Genghis,
I'm not sure I understand your argument on noise due to the high RPM.
The shaft speeds will put all the compressor and turbine blade passing frequencies effectively out of audible range.
Jet mixing noise should be low if the maximum work has been extracted by the power turbine prior to exhausting through the nozzle. And, as it scales with 7th or 8th power of jet Mach no, it is usually insignificant on turbo-props compared with pure turbo-jets where the thrust is generated with jet velocities of 300-500m/s.
Turbomachinery generated broadband noise may be relatively more significant, especially with the shorter blade chord length. I doubt that it could be dominant.
Combustion noise can only usually be heard at low power settings (e.g. the occasional hoot of taxying airliners).
That doesn't leave much but propeller noise which should be independent of the powerplant.
I don't have much experience of small turbines except hearing APUs running on the apron. Do you have any more specific knowledge of the noise sources?
I think that specific fuel consumption is probably the biggest problem at present (and cost).
M