For what I have tested , I have to go along with Matt.
Once planing, I was a bit faster on a 85 wide slalom board.c/w the US147.
But then again, to win the race, I would choose the US147 because I would gain a lot after each jibe in 8-12 knots. And the difference for heavyweights would even be bigger because for them it is not a matter of being the fastest on the straight line but instead making sure they plane fast again after the jibe.
Anyway have fun testing and let us know how it was.
IS87 / IS101 / IS122 / US147