Qjet is a 750 and in stock eliminator form makes 470+hp on dish 350 and 440+ on 305. The last stock mercruiser tbi we did made 237 with the stock boat tbi set up and 276 with a 750 holley and locked out distributor with 34* timing. Only change was intake/carb/distributor on a fresh built 350 tbi mercruiser.
Something sounds off on that TBI mercruiser. They are factory net rated at 300-340 hp depending on the unit. I have personally watched a stock 260 hp rated 305 L30 vortec engine with TBI crank out 310 hp on an engine dyno with no accessories on it and 1-5/8" primary long tube headers. 30° total timing and 12.6 air/fuel ratio.
I tuned a MEFI 4 ECU on a 305 Vortec with the 395' cam that put 270 hp to the ground through a SM465 and 12 bolt in a C10. 305 had a dual plane 4bbl intake, factory marine tbi adapter and factory 2" bore marine tbi. Headers and dual 2.5" exhaust. Same engine that was on the engine dyno.
The Q-Jets I build are 850 cfm rating. Everyone mistakingly calls them 800 cfm but properly adjusted they are 850 cfm. Made no difference though. At 350 hp the CFM of a stock 2" bore marine TBI was more than adequate for the 350. I have made 370 hp at the wheels with a 2" bore TBI and spun it 6,600 rpm. Was still not restricting the 350.