So your sail has leech that is more straight. My F16 sail has the same althought I have the same area as the Fx-one on a similar 8,5 mtr. mast. The behaviour of it are the same as you give for your new sail.
Although my main was not specially designed for heavy weather.
The reason my sailmaker gave was that a larger squaretop requires a straighter leech to perform. Can well be that your older sail had to much leech curve for the squaretop it featured.
But no matter what the reason . mr Ogletree made you a fine sail.
Wouter
Just thought I'd add in my two cents as well. I can't really say much with any sort of authority as I haven't done much cat (uni) rigged sailing at all but I've done a lot sloop rigged. My immediate thoughts were to the mast rotation.
I've gone through the same problem myself. Boat has been going like a rocket but there was no height. I sail predominantly in a [color] really shifty place (Canberra, Australia) so height doesn't matter so much as long as you are in the right place at the right time and going the right way. When it came to sailing against other boats in more stable conditions we would go like a truck but would loose some height.
My thinking previously went as follows:
Other boats pointing higher - must have more power than me
Ease downhaul, move mast to 45 degrees ish (most stiff) and grunt the whole boat up.
Problem was, the only way I could get the boat to really move was to ease a fair bit of sheet and head down a bit more.
This turned out to be the wrong way to do it (in fact opposite). As soon as the airflow did attach to the whole sail we got an enormous increase in power that was only tipping us over and not pushing us in the right direction.
When you want to point higher and the breeze is stronger you want a flatter and flatter sail and generally the smaller the angle of entry the better. I've found that having the mast all the way in, and I'm talking between one and two inches from the boom, makes the boat rocket upwind and the pointing has returned. Also, because you only have the weight of one person and the lighter weight of a cat rigged boat to push through the water you need even less grunt to do it than with a sloop rigged boat.
I would try bringing in the rotation and possibly the traveller as well. When the breeze kicks start depowering with the downhaul until it maxes out and then start easing the traveller out when you've got the rig as flat as you can.
But that's just the thoughts of a sloop rigged sailor but as it is a forum I thought I'd add something.
...and that's my two cents 
Hope this helps.
Andrew-
I think this is worth trying on the cat rig but is probably more beneficial on the sloop (IMO especially with wingmasts) since the minimum rotation really opens up the jib slot (and probably helps "stiffen up" the forestay since the mast chord is now more nearly aligned with the forestay so the jib would flatten as well), aligns the prebend more with sheet loads so probably get more induced fore-aft bending and mainsail flattening that way, and allows the squarehead to bend off easier. I have found the Taipan squarehead is fairly small so mast rotation seems to have less effect in this last regard than on other, larger squarehead sailplans - such as an "A" (which also has a taller mast with more mast above the hounds). I have been told that one "trick" when sailing in high winds and using minimum mast rotation is to ease outhaul (this is on a unirig- Don't know how it would work on a sloop due to potentially closing off the slot some)- The rationale being that the top of the sail will be "blowing off" anyway and you will be sailing on only the lower portion (below the hounds) so you want to power it up some to compensate for the loss of power up high.
Kirt
WOuter, Andrew and Kirt,
Thanks for the info,...it is a nice thing to get tips from around the world!
I always increased outhaul on the old sail(..it was so beat that the only part of the sail that had any life left was the bottom...ha!)
The sailmaker, C.Olgletree, stated an interesting adjustment,...which was opposite of what I was doing....
...that was LESS outhaul in heavy air,...NOTICE- this is a custom sail,...not a stock sail,...so tuning my be sail specific and NOT general sail physics...
....it is apparent to me now,..that he built me( weight, age) and my area ( 15+ steady, waves 2-3) into the sail.....which is an allowed feature for the T 4.9 but not for the I-17,...for my sail would put me out of a class race because it is not a factory sail........but there is no other I-17 for about 3000 miles,...so I am not going to worry about another I 17 at the start line...
...so Mr. Olgetree instructions are a mix of what we all thought,....reduce rotation + 1 inch chord on the foot ( in respect to the boom)....sets this sail perfect....
Bruce
..ps,...a great forum for me,...thanks
Bruce-
Glad we were able to help! So Charlie's basically agreeing with the last suggestion in my post about easing outhaul to help power up the bottom of the sail (since the top is "blowing off" you can essentially discount the effects up there- the minimum rotation helps ensure this blowing off) to compensate for the loss of power from the top of the sail and the flattening effect of the max downhaul and minimum rotation.
So how are you doing against the FX now?
Kirt
Kirt and all,
1. Just got back, was in Sweden, Finland and Russia for 3 weeks,.....man is it cold there or what...!!!,....i have a study group with 45 other professors,..we look at different countries education systems,........Finland is #1,..Sweden #3 in the world as far as this topic,....Russia,...surprizingly has recouped with a great system,...my opinion,..Russia public education system, at this point in time,..turns out a better well rounded student than the USA,...mainly because of the emphasis on dance, music and physical education, along with academics...USA kids don't do the physical stuff like the previous generations,...oh well..
...see new post about FX
Bruce
Bruce,
I've been sailing an I 17 Euro (with a spi) in the UK for 3 years now and I would like to make the following comments about sailing it up wind.
It's difficult.....!
With regards to what others have said above. Agree with most of it. Here are my comments specific to the I17.
1, Once you are getting over powered, you must downhaul hard. I have slightly altered my downhaul system to make it more responsive and allow me to get the sail down very hard (Around 1 inch from the gooseneck when it's really howling !) but still staying with 8:1.
2, As the wind comes up, so do the plates ! This keeps the boat driving and in control and stops you 'tripping' over them.
I use the following as a guide : Once on the wire fully, plates up 3 inches. then move them up another 12 inches at 15 Kts until it's getting windy (+20kts) and then I'll only have around 12 inches of plate in the water, when it's getting wild (25+) i'll aim for 4 - 6 inched of plate down !
3, Mast rotation. Max until you are on the wire. Once wiring but needing power, spanner pointing at the shroud, then move slowly back as you get over powered to around the back beam when its windy (20+).
I never play with the pre-bend or diamond tension. I have my spreaders set with about 20mm of rake and set the diamond tension to induce around 1 inch (25mm) of prebend (at the spreaders) in the mast.
Oh, and now you have been sailing the I17 a while, have you investigated a Kite ?
It makes the boat a scream down wind - and as fast as most 2 handed bigger boats.....
Simon,
thanks for the post,....see the new line '...pointing like a bloodhound"
...I need to hear what you have to say.
regards,
Bruce
OK, Bruce,
Had a look. at this :
Was that using less plate as discussed previously ?