I re-read your initial post this morning with my first cup of coffee and there are some other observations that might have some merit. First, are we sure those "rebuilt" distributors are advancing properly?? If they are properly done and you are not able to coax more power out of the boat by doing the "power tune" (a term I've never really heard of before, but having fooled around with motors all my life, is quite descriptive and I know what you mean!) then I tend to think there's something other than the timing involved.
I know there may be multiple things working together on old boats, so be on the alert. When you rebuilt the carbs, did you change out any jets, etc? Just wondering. Are you sure the linkage is allowing the 4-bbl to open fully?
I learn something every day, and I'm not familiar with a 33' Commander, so I'm thinking you either have a 31 or a 35. Either way, my experience with the Q motor on this size boat is all good. THey're great motors and they'll make either boat run like a bandit. My family has a 31 Sedan with a pair of the 327Qs, and that boat will fly!
Next question, have the cylinder heads ever been replaced? The Q cylinder heads are high performance, and it would be unfortunate to have them replaced with "taxi cab" heads during what some yards do with their "exchange program". If the heads are original, then I'd look at those solid lifter valves. Valves set too tight lead to two running conditions, one...the boat won't want to idle down smoothly, and two....it won't develop full power at speed. Be sure they're set hot to spec.
Here's something for your files, and something that could help too. It's the Eaton Troubleshooting Guide, and although they used Ford motors, this guide works just fine for any solid lifter V8 of the era, becuase they're essentially all the same.
http://www.network54.com/Forum/424840/message/1123099503
Now for those props. I think Tom has a good hunch. Did the previous owner happen to change those props out, perhaps after doing damage to the originals? I wonder if he thought he would get a faster boat or better fuel consumption??? In any case, an over-propped boat will NOT perform well, and your symptoms sure sound like this could be the case, especially after you've done all the work on the engines and obviously know what you're doing with a wrench!
Last question. Fuel filters? Have they been changed out? You probably have not found it yet, but there are some posts that indicate there have been more than one boat suffering from clogged fuel delivery "from the tank". The shut off valves, or the output line from the tank to the filter, then to the pump may get clogged. This is especially true with the guys with newer boats and anti-siphon valves, which our old boats don't have.
All the best, welcome aboard, send photos of your boat if you have em! (you can email them to the address on the forum, which comes to me, or you can post directly if you are able to upload to a photo host directly)
Paul
forum moderator
1966 38 Commander Express
FXA 38 3004 R
Original 427 power