KC, I think that there are some places that can reprogram the Merc EFI system for you, and they should also know what the limit is on the stock injectors. I would call AZ Speed & Marine (in Phx), describe your engine mods, and see what can be done using the stock EFI system. Larger injectors are a viable way to increase power with your other mods but you will need to have the ECU reprogrammed to match. I'm not familiar with the manifold that Merc uses, hopefully that won't be a limitation on airflow.
A single carb setup would easily support the power you're looking for but obviously you would lose some of the benefits of EFI. If you choose the carb route, you'll need a new intake, carb, fuel pump/regulator, flame arrestor, and miscellaneous wiring/plumbing mods to make it all work.
Couple other thoughts for ya: Are you running stock exhaust? Headers or good manifolds (with individual runners) will help get the power you're looking for. Also, you mentioned new cam and heads. Make sure that your valve springs and the rest of the valve trane are compatible with the cam you've selected.
What heads and cam did you buy?
Good luck!
-brian