Ok, Since my boat is being fixed, I'm going to re-vamp a few things.
My mainsheet is big, fuzzy, and gets heavy when soaked. it's 6 or 7:1 purchase.
I've found some pictures of tornados or even a M20 running very thin sheet material. Appears that they run the thin stuff through the blocks, and put a cover over it (tapered) for handleing.
The M20 even had a 12:1 purchase!!! Using very small looking blocks.
Anyone have any insite into a mainsheet like this? I'm sure the cordage is $$, but if it's lighter, stronger, and allows it to run freer (sp?) I'm game.
Look for tapered lines at the usual suspect online stores. I don't know if catsailor has them, but layline does, as does aps.
Could be Warpspeed with the cover pulled back. That is how I run it. Just as strong with the cover off. The fuzzy stuff is for...... Well you know.......
I used to use the old 3/8" fuzzy trophy. Yuk!!! The Warpspeed handles just as well in the 5/16". It does not get heavy or hold water either. 2 more reasons to buy.
The stuff runs through like its namesake too!! Super fast and smooth.
AJ
Nacra 6.0 Express
I'm checking on that stuff now. Also wondering about using smaller blocks (40mm carbo's, or even 20's?), and going to a cascading type system to get higher purchase.
Any info on how they got a 12:1 system out of the rigging on the M20. Looks like part of it travels up the boom (I don't have one), maybe it's a 6:1 with a 2:1 cascade on it?
I dunno, maybe I'm just looking for ways to spend $$
I just did that custom job on my Inter-17R Mainsheet, the change is breathtaking.
I'll post a pic and exact sheet names later.
What you do, basically, is choose a mainsheet with a spectra or vectran core, and 'pull away' the cover. The exposed core runs incredibly smooth in the pulleys. Also choose a sheet 'cover' that does not 'expand' or 'sponge' water in.
I'll post a pic. I was inspired by som.e canadian Olympic Tornados and 1 of the 18HT at our club that was bought from the Alter Cup.
Later 
I changed to the Carbo Ratchematic this year and bought 3/8" Swiftcord. It runs through the blocks beautifully and has a great feel to it. There is no cover on the line and it does not absorb water. I recommend this if you buy new line.
keep in mind that your class rules limit the amount of purchase you can put on your mainsheet. On the Nacra 6.0NA, it's 8:1 (although the factory system is 7:1). I'm not sure about the 5.5.
According to the NACRA class rules, the maximum purchase on any NACRA mainsheet is 8:1, no cascades allowed.
Getting the full 8:1 is very important for a 5.8, and I would figure for a 5.5 too.
Our N6.0 mainsheet's is 7/16" trophy braid. The previous owner didn't wear gloves therefore he choose that line type and size. The line was too big to run smoothly through the blocks, especially on 0-5 wind days. I stripped the cover off for the part of the line that runs through the blocks. It runs much better and even cleats better. I need to take off about another foot or two of the cover because when we are going down wind the sheet is often being cleated right at the taper. I was concerned with the strength of just the trophy braid core. However, we have not had any problems with it to date. The strongest winds we have been sailing in with the tapered sheet is around 15 consistently with gust just slightly above that.
I've had this issue also, even with my brand new XLS 7/16th sheet. My only solution to the problem is to arch my back as I'm crossing under the boom. (I know by that point its to late)