All direct injection engines suffer that issue TBH. Very common on various VAG TFSI's
I cant really see any solution either. If your not flowing petrol over the valves, and your pouring ****e into the inlet from the crankcase, craps going to build up.
Having said that, have you ever looked inside the intake of a diesel engine thats done any sort of mileage, with EGR etc? They get absolutely filthy, but it doesnt seem to affect them too much.