If you're even contemplating a 250XB,....Gas shouldn't even be a concern.....guys who buy Ferrari's don't care about mileage!! That motor is built for speed and top end,.....you will be one of the fastest boats on the water with that motor on a 929,..AND you will always be at the gas station!! LOL....It's a GREAT motor,...but it's not as good on gas as an OptiMax.....you will probably be looking at @ 2-3MPG if you're lucky...probably more like 2!!
The Opti's get about 5-6MPG, and that is significant when you're running a great deal.....it's very noticable!!
Good luck and if you go that route,,.....you might want to consider pursuing a gas sponsorship!! NOW!!!..LOL
Good luck Chris,....seeya.