Dont think it will be the props, unless she has had an accident that required 2 new ones and they were the wrong pitch.
Going by what you say I would be looking at:
1. Fuel problems- dirty filters/seperators not allowing enough fuel through
2. Check the air filters
3. Are the Turbos coming on line- could be that the dump valves are stuck or passing
4. May be a throttle sensor is not working
5. ECU problems (if fitted)
Personally I would go along the fuel route first. On twin inboard engines, especially the smaller HP ones if one engine is down on power then at times the other engine cannot pick up the pace and get enough revs to allow the turbo to come in.
With blocked filters on an inboard it is common to loose top end revs/power as the filters get blocked. The engines will rev freely to WoT with no load but will fail to achieve WoT with load due to fuel restriction, the worse the blockage the less power through the range until it stops. I dont really know much about the Steyr engine, but from what I can remember they are fitted with various sensors that can affect the operation.
Hard or Soft it's never BIG enough