Keef
That's exactly what I meant. Same prop, same RPM, must be same HP being developed. No way around that at all.
But re-reading the original post, I noticed the aircraft are not necessarily identical! They are referred to as "X" and "A".
This of course makes nonsense of the original question, because (obviously) sticking a powerplant (engine+prop) which shifts 180hp's worth of air backwards on a 450kg plane will make that plane go a lot faster than sticking it onto a 1400kg plane.
But surely everybody can see this, so the original question must have presumed identical aircraft... which takes us back to what you and I said: there cannot be any difference in performance if the props are the same and the RPM is the same.
It becomes obvious when one knows how a VP prop works... open the throttle -> more power coming out of the engine, and the governor (seeing the engine trying to increase the RPM) coarsens the pitch to maintain the RPM constant while absorbing the higher torque.
Flyin'Dutch'
Trying to separate torque from HP does not mean anything because HP is simply RPM x torque. The original post says 2550 RPM in both cases, so the torque must be the same is the props are identical.