[Bimmer]

Having been to quite a few airshows, I can't say that I recall seeing any discernible smoke behind any of the planes, certainly nothing that would be visible over any distance. That said, if you have noticed smoke at low throttle levels, remember that these are highly tuned engines that often are designed to run at the high end of their rev range. Race car engines are like this (or they were before computerized engine controls) - at lower speeds they often spit, cough, smoke, foul plugs, etc., but at full tilt boogie they are smooth as silk, burning their fuel-air mixture more completely due to ignition and valve timing being tuned for those conditions. The kind of inefficiency in combustion required for dark black exhaust smoke is quite significant, and certainly not something I would expect to see from a well-tuned engine running in its optimal range.

As to boost systems that actually inject something into the cylinders, that's a different story. Methanol-water injection is used to to compensate for low-octane fuel being used in high-compression engines, in order to prevent premature detonation. It certainly makes sense that this would produce some sort of change in the exhaust, likely a sort of combustion byproduct particulate-laden steam, as the water picks up bits of carbon from the piston top.