Suppose I have two boats whose Portsmouth ratings are very similar:
Hobie 17 (1up)
H17 74.2 78.5 76.3 73.9 69.7
Isotope (Sloop, 1up)
ISTP 74.2 78.9 74.3 73.8 68.1
Can I safely assume that these boats are equally fast?
Jonathan
Depends on the sailor 
I know that I was outrunning Isotopes on Jordan even when my centerboard was dangling off the side of my starboard hull.
But if you were to sail an Isotope, then the Hobie 17 on the same course, could I assume you would be about equally fast based on these ratings?
Jonathan
If I knew how to properly sail an isotope, then yes, that would be a fair assumption.
In this particular case, in lighter winds, an isotope would fly by a H17 just because its so light. In heavier winds, the story changes a bit. Also, throw in some chop, waves, and other environmental factors and you'll begin to see the differences arise.
If the Portsmouth Rating number is lower, the boat is faster if all other condition are the same (skipper with same ability and so on). The H17 is slower compared to the Isotope in higher wind speeds. If we are to believe the numbers. 
Eh, I can tell you right now that those numbers aren't right if thats the case.