I know that both turbocharged & turboprop engines offer better performance above 10,000 feet than normally aspirated piston aircraft. My question is to what degree do their optimal cruise altitudes overlap?
Which would be more efficient (in terms of both fuel and time) for flying at 17,500?
What about 25,000?
As discussed here, piston engines are more efficient from a strictly power based comparison. Since you ask a slightly more specific question lets look at some operational numbers.
Turbo Charged and Turbo-Props are completely different engines that work on different principals so comparing them might not be totally fair but anyway onto the numbers.
There are few piston aircraft that fly above 17,500, the Piper Matrix being the first that comes to mind thats currently in production and has a comparable turbo prop counterpart. At altitude it burns about 18 gallons per hour. We can compare it to the M500 which is their turbine offering burning just under 40 gallons of Jet-A an hour. However one is running 100LL while the other is running Jet-A so Im not sure you can directly compare the flow rates. From a cost perspective Jet-A tends to be cheaper on the whole than 100LL so your fuel cost per hour could be comparable depending on where you buy your gas. Theres fairly cheap 100LL if you fly to smaller airports so the Matrix is going to eek out ahead on that metric.
The PT6 in the M series turbines tend to have longer lives than the TIO-550's in the Matrix but again your comparing spinning and reciprocating engines which are operationally quite different.
Also of note, since no country is tagged here. You cannot fly VFR at 25,000 ft in a powered aircraft in the US. More practically its highly unlikely any insurance company will offer a policy on a turbine aircraft like an M500 or a big piston aircraft like a Matrix if you dont have an instrument rating.
