Symptoms: low max rpm's (5600), low max speed (48 speedo), relatively low compression (105, 105, 107), intermittent runaway throttle.

Attempted solutions: rebuilt carbs, top end, exhaust leak
All along it was:

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blown out rear seal. Found a piece of it in bottom of hull back in July but didn't register that's what it was. To give credit where credit is due "w/rpm 500" did mention bad rear seal as a possibility.
This explains all the symptoms, huh?
(Just so you all know this is a six year old SBT motor.)
Thanks for all the help along the way guys. Now please excuse me as I have some cases to split.