Actually, supercharged piston engines do loose power significantly, once past 'rated altitude'. With any boosted power system the throttle opening is limited by the induction pressure, the cylinder strength, if you like. As density decreases, the throttle can be opened, until, at 'rated altitude' it is wide open. After that the engine loses power as it climbs. The limits were reached back in the mid '40's, around 50,000 feet and 500 mph indicated. Gas turbines, conversely, will always pump just what they need, so long as there is air to pump. They do increase thrust as their speed increases, due to both ram effect and the lower air temps allowing more fuel to be burnt as they climb. At 50,000 feet turbines are limited by the skin temps of the aircraft carrying them, not the lower density of the air. So there are huge differences in the operation of the two propulsion systems. Propellers are very efficient, up to 300 odd mph, after that a turbine will be favoured. Take a look at your local airport and you will see very clearly the sort of division I mean.
Evan, WB #12.