Hey Big Bird,
Been busy, my boat motor blew up, that's a $10,000 problem. Working on tuning the vette, and finally got 3 days in a row without rain, so taking the kids to the beach...etc...
Anyway, are we talking stock or aftermarket? Essentially, all you can do with TBI is increase flow, put in a plate, and that's about it. You could work the intake some, but IMO, it is kinda limiting. Now, that does not mean you can't make big power with a TBI setup, in place of a carb for example. But in terms of stock setups, not much.
I just did a go fast boat with a Merc and TBI. These injectors are only available now in remanufactured from Rochester. They are the same as found in many GM vehicles. They are expensive, about $100 each. Choices are limited.
If you're talking aftermarket, that's an entirely different case.