Throttle body fuel injection is by far the easiest way to adapt fuel injection to "late model" MGBs, ala Moss's kit or something similar. As for whether or not there's any noticeable power gains, I've no clue. I've yet to hear first hand experiences from anyone that's installed the kit on their car. However, I don't expect there to be much improvement on an engine that's other wise stock.
However, I'd like to play devil's advocate here, and say that you CAN adapt multi-port fuel injection to an MGB despite the siamese ports on a stock head. You'd just have to over come that obstacle by either putting two injectors on each port or by putting a single injector in each port and sending multiple injection pulses (2 per revolution) to the injector.
Granted, like Kenny said, it'd be a lot easier to do multi-port injection with one of those high dollar cross flow heads.