Im not sure what chargers your running?? My M-3 will sit around the 2 PSI mark at about 3100 RPM, to me the boost should be coming in sooner then 4000 RPM (granted I dont know what pulleys/blowers your actually running) Whats the back of the boat look like? Soot?? What do your plugs look like?? Have you watch timing on a timing light to see how it reacts?

It has to be an ignition problem or a fuel problem, if the motor is sound inside.
Didn't read all posts, but did you do a compression test hot..
I had a problem with detonation and it broke the ring lands on the piston before. It wouldnt show up on a cold comp test.

New fuel in tanks this year and also ran two separate 5 gallon tanks just to see if anything changed - no difference. We checked the knock sensors while @ 4000 rpm all everything checks out

I'm running the M1-SC chargers. According to the dyno sheets I see 2.3 lbs @ 3000 and 2.8 @ 3500 and 3.6 @ 4000.Not sooty on the transom and I pulled all of the plugs on one motor and they look perfect Also checked timing and that was dead on as well

i think I've ruled out fuel running separate tanks to test. What would I look for as far as an ignition issue? Didn't do a compression test but did do a leakdown and all cylinders were 5% or less leakage.

Hmmm definitely very confusing for sure... the only time Ive seen a similar issue with my boat was when I was trying different props this year, Just simply wasnt putting out the boost or pulling the proper RPM... swap back the props everything was good. You havnt changed your props though at all correct?

i haven't changed props, been running the same ones for a few years.

How about full throttle travel at the TB?

Yes checked that too, fully open @ WOT. The thing that's just so strange is they both tan perfect on the dyno and now they're both not performing. Both run great up to 3500 rpm, you'd never know there's an issue until you push the throttles. I'm putting on two smaller props today to see what it does but I'm guessing it'll run right up to 5000 rpm but won't have any boost. I'm going to run it with the Prochargers connected and then disconnect the belts to see if there's any change.

Thanks everyone for all the suggestions - I've gotta get this figured out!

I can't wait to hear the solution! This is a head scratch-er for sure. Keep digging, the season is too short to be head down @$$ up in the bilge all summer!