the R-44 has power well beyond the MP limitations
This has been a puzzler for me but i've not been there.
The secondary reason not to exceed any flight manual limitations would be insurance cover.
Can anybody tell me why they don't use torque meters in pistons?
It's a good question, my guess would be that the information isn't supplied for 'free' by the engine and MP + rrpm droop had become the industry standard. I believe the turbine measurement is derived from an oil pressure effect/sensor. I'm sure someone who knows will explain this? One curious, random-internet-found diagram follows!
http://www.aircav.com/img/cav/inlet.gif