Brian - From a completely non technical point of view I'll say this:
In light airs/choppy water the square top seems slower, in flat seas it's about the same.
As the wind started to fill in a Victor (13kts) and we were able to stretch out on the wire the big head sail seemed to be faster, I sailed over the top of Brett and gained about 100m on Todd in one upwind work.
I'm going to go out on a limb here and say that this is probably b/c it's much easier to oversheet a squarehead in light/med conditions than it is a pin head. Oversheeting is a painful death in light air.
