If mx is checking the compression at every 50 & 100hr inspection (Im assuming used for hire and working on FAA regs, not sure what you're local regs are on inspections), I would think that would catch any drop on the entire engine well before it became a problem. If you're losing compression faster than that, theres got to be significant damage going on (likely uneven between cylinders too) and I would think it would manifest itself as roughness in addition to power loss.
How long are we considering prolonged, with respect to ground runs at high power? I was taught if the plugs don't clear noticeably in a 20-30 second "burn" that it needed looking at anyway.