The AA's?

I've got mine maxed out on my 95 right now. The accelerator pedal feel is AWESOME, but anything past about 1/2 throttle at 2000 RPM+ is dead space. I'd call it 60 HP by the seat-o-the-pants dyno. Basically, all you get with tuning on the OBS trucks with stock injectors is fuel down low and more power before 2000 RPM. Not saying that there are no gains above 2000 RPM because that isn't the case. With some SOI and injection pressure, you get gains up top, but it's nothing to write home about.

I have always felt that the Powerstroke's just seem to level off around 2/3 throttle anyways, with stock sticks.

Is there any explanation for this, or just a injector volume thing?

The proverbial nail on head.

AC's can solve that with some tuning though. Lots more volume.