I'm not claiming to be an expert, I'm only relaying my own personal experience with trim tabs. I would never steer anybody wrong.
For what it's worth, and I havn't even mentioned this yet, I've been dealing with a similar porpoising issue with my new 16' Smokercraft Ultima. I have a 75 4 stroke Merc on it and it's basically ran like crap since it's been in the water this spring. Porpoising is the biggest issue.
I've had it at the Merc dealer in Rouyn Noranda since our last trip up.
So far they've adjusted the height of the motor, changed the prop and even installed a wedge behind the motor.
Currently a hydrofoil has eliminated the issue but that's only temporary. I'm 7 hours south and the price they quoted me for trim tabs was ridiculous. I'll take the hydrofoil off next time I'm up and install the tabs myself.
Until then at least the boat is functioning semi normally.