With all that experience Robert I would have thought (guess I was wrong) you would know that a Carb'ed engine has fuel flow over the valves which cleans them up and a DI engine is a "dry" intake system with no cleaning action at all.
And this is why at least two manufactures are that I (now) know of are going with a duel injection "wet" system.
Lets be clear here. When manufactures are talking about carbon on the back side of the valves they are not talking about a little coating here. We are talking about a major build-up. We are not talking about worn out guides that pump a lot of oil onto the back side of the valve.
And one more time. I wanted the OP to know that turbo failure is VERY unusual for the EB engine. Ford has very good turbo's on this engine. Unlike the 6.4 or the first gen 6.7 engine which were junk these turbo's are good. Now if someone did a de-carbon to this engine then all bets are off and this is where the trouble starts.