I believe the decompression lever on the 2GM20F just pushes the exhaust valve down some so it is open. I tried this once on my 2GM while idling... and it immediately snapped closed--I don't think it is meant to be messed with on a running engine. Then again, I doubt you hurt it.
No one else has mentioned this, but it can't hurt to check your valve clearance. You sound like someone not afraid to mess with stuff
and even if it has nothing to do with your current issue, it would be good to know the clearances are correct.
BTW, I've read the advice to run the engine at 80% etc. Not a bad idea to do that now and then, but you should cruise at what's comfortable, I think. My boat has the original factory 2 blade prop, and under most conditions 2500 rpm gets us somewhere around 6 knots, IIRC. We rarely use more than 2500.