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.

However - Todd did win the series with a pin head sail - it should be mentioned he is an excellent sailor with 15+ years experience on the boat (I hope he doesn't get a big head sail!)