I can believe that it's possible to improve performance on an aeroplane engine through modifying the exhaust - although whether or not the performance gains claimed are realistic or cost-effective is a different issue!
I'm not claiming that the V8 engine in a Porsche 928 is in any way directly comparable to a flat 4 or 6 cylinder aeroplane engine, but a re-designed exhaust system for the 928 unquestionably increases measured HP on a rolling road (without any change to ECU or fuelling). I have definitive proof that this is the case. For sure, it's small (around 5%), but it's certainly achievable.
The improvement is obtained by reducing the exhaust back-pressure, thus helping the engine to "breathe", by replacing the stock manifolds with high-flow headers and / or replacing the exhaust centre section with an X-pipe to increase the speed that exhaust gases are removed from the engine. Both of these are at their most effective at high revs, so may not be applicable to the "lazy" low-ish RPM regime of most Conti-mings.
FBW