Sudden Loss in MPH

This is my second post. The CCF community solved my last problem, so here we go:

1992 Excel with a 351W.   I bought this last fall and I have had a string of issu)s despite sinking mucho dinero into the engine this past winter (rebuilt gt40 heads, new msd ignition (but remains carbuated), new Edelbrock pro intake, new Holley 4160, new exhaust manifolds, etc. Had the boat out a few weeks ago after all the upgrades (and solved overheating issue) and the thing ran like a scalded dog. 47mph WOT, and 42mph WOT with footer behind boat. Two weeks ago I lost some power. To be more specific I lost 6 mph off the top end, and the lower end is absolute dog crap. I can barely pull a footer out of the water - takes at least 20 seconds to plane out. Here are the symptoms:
1. Lost 6 mph off the top
2. Boat not as consistent with turn key starter
3. Engine has backfired twice
4. Engine reluctant to start after warmed up and shut off.

It's almost like the motor isn't getting enough gas - that it's being limited. I checked the carb inlet valve - it's clean. What all could I be dealing with?

Yep. Seemed great then not.

Have you rechecked the timing?