Fanch,
I use the GoPro on an F450, and a Sony NEX on an S800. You are correct; the quality of the GoPro does not compare to the quality of the other light weight mirrorless interchangeable-lens cameras (Sony NEXs, Panasonic GHs, Sony RXs, or the new Canon). I have friends that use a F550 with stock motors and 1038 props and the Sony NEX-7, which is the rig I am currently working on as a midway between my F450 and S800.
As far as the GoPro, it is a nifty camera that seems to do the trick for basic aerial photography and video, and I have friends using that for their business just as Tranquilla is. They can't charge the rates compared to my friends that use the Sony, but none the less they get plenty of business.