I think 5-7 MPH is a realistic estimate of what you'll gain. I went from a 365HP 454 mag to a custom built racing motor that turned 615HP in a 1990 242LS. It was the 454 block bored out with aluminum intake and heads... so the engine weighed less than previously too. I went from 54.5MPH top speed (on GPS) to 71 MPH. Thus, I gained about 1MPH for every 15HP added. You'll probably need to go up 2in of pitch on your prop too, so add that to your cost evaluation.

The EFI is very nice on cold starts. Otherwise, I have no preference between the carbed motors and EFI. If you get the choke set right, it doesn't really matter.

Also, you might want to confirm that the motor mount locations won't give you trouble if you convert.

Long story short, Formula's ride great, but they take a lot of HP to move them fast.