imco extension box
 

I have a 2000 formula 271 fastech (stepped hull) hull is blue printed, imco steering, imco sc upper and -1 lower, 540 engine, 650hp, 750 ft.lb., turning a bravo1 labbed 29.38 pitch turning 5100 rpm @ 80mph gps. I was curious about adding an extension box between gimbal and transom, was curious if anyone had tried one on this hull and if it would gain any speed, also have a prop shaft height of 3.75" below hull. If the ext. box is a good idea which would be better the neutral box or 3" offset box?