In short, moving the pump back also moves the biting edge of the Intake back...thus less Hull wetted surface at WOT. Hopfully the boat has enough WOT potential to benefit from the setback in the first place.
I did my setback...there are several pics somewhere on this site.
Not only does my 21 Daytona pack air better, it goes a little faster up top and one of the biggest positive factors for me...more room in the boat as the rear bench seat is now some +6" back.