One board will always bring compromise. His top end is 20-25, so one assumes that the rest of the time, will be at lower wind speeds.
Racers will know its "go big or go home" and I think Naish once said "there's worse things than being overpowered".
Even if he is not racing, he may well want to be planing for the maximum amount of time and an IS96 with 7.8 & 6.3 wont achieve that. I agree the 133 may be slightly big but he needs to be closer to 133 than 96.