now I'm no scientist/engineer/mechanic, but to my understanding, the FUEL isn't the problem with DI carbon buildup, it's where the fuel is delivered.
with port, or even tbi, the fuel "washes" the backside of the valves.
if it's really bad, you'd be better off figuring out how to add a single injector after the throttle body...
you'll notice some manufacturers, while never acknowledging there was a carbon problem, have redesigned the next generation of those engines with both port and direct injection...