I'll have to stick by my assertion from before, I'm just not seeing any carbon buildup, nor any other deposits accumulating on any EFI engines to the degree seen on older carburated engines. As long as the engine is operated for a long enough time for the engine to reach operating temp and actually driven at anything approaching a normal RPM range then you shouldn't have any problems with any foreign buildup, either in the combustion chamber or in the catalytic convertor (exhaust).
If there is actually a buildup of either carbon or other particulate in the engine, or exhaust then I'd suspect that it's not actually a problem with how it was driven, but more likely a problem with maintenence, ie. oil change intervals, leaking injectors, etc.
Of course, *if* the engine is literally only run for a few minutes at a time, or continually short tripped, then you will encounter problems, mainly due to the buildup of condensation internally which will destroy the oil in short order. But again, that's more of a temperature, or maintenence issue than it is a rpm issue, and no ammount of full throttle blasts will cure that.
I would have to wonder though, that if you're seeing smoke exiting the exhaust during full throttle blasts, that perhaps what you're seeing is actually excess fuel, or more likely burned oil, than carbon and rust deposits. Carbon and rust deposits really aren't smoke, nor to they resemble smoke upon exit.
End result, as long as the engine is kept in top shape, and driven normally, there shouldn't be any buildup of either carbon or rust to a degree that would hinder engine performance/mileage.
Jay