We as humans feel sluggish in this heat so do these motors. The heavier the boat the more noticeable. All the DFI motors have a parameter of self adjustment but it's a small window. Steve suggests a smaller prop, I suggest Nitrous
A good example of this delima Saturday with a Gambler/250 EFI, owner couldn't break 63 with a worked 26 Triton. I punched quickly into the 80's earlier in the week, same set-up, different lake, different air and water temp. Now this is a great speed/acceleration difference here (but remember, I've been racing boats since the early 80's). The Gambler owner borrowed a "28" and a "27" (the boat's been spinning a 25 Trophy that Steve worked and runs good for a Trophy I might add) but my 26 Triton out accelerates it hands down. 28 ran 70mph @ 5k, 27 ran 72mph @ 5k (these are typical #'s for tall pitched props this time of year).
A proven test year after year with boat owners:
Dyno results deminish after a few pulls, reason: wet air around the motor. Steam is created as the motor warms up, and on a 250, there's at least a 10% drop in hp output.