I would only use it for servos, but I don't really want servo jitter.
What I want to do is not controlling the whole helicopter.
It has been done well for quadcopter drones.
That just has motors, and no servos.
There was one reply from a guy on the Ardupilot forum, who did something with helicopter thruster commands, but I suspect they have actually said their piece:
"Read the documentation"... "Practice". - seems like lots of typical c++ developers! 
I could try rewriting Ardupilot software, but I hesitate
to "poke the dragon" in this case. 700k lines of code is formidable.
I would rather build something similar that runs very much "unix" and has a lot of separate pieces.
There is a helicopter app referred to in the YouTube quadcopter article which looks pretty good.
You probably don't get a real detailed Gyro/GPS/Nav setup, though. You couldn't store it on the Arduino boards.
I could go Raspberry Pi if all my inputs could be made digital, but the I/O system
is not very intuitive.
Microcomputers like BeagleBone and Pi run around 1 Ghz.
Once I get the actual heli built, then I will have a potter around with the Arduinos,
and maybe get an MKR Zero. The helicopter will be 2 kg or so, so room for a fair amount of processing power.
At the moment, I am prepping my conceptual design, and looking at suitable apps and bits of apps.