RE: prop spinning too fast?
NASA has tried the whole supersonic prop thing.... it doesn't work. Look into the data, it clearly shows that props are more suited for low-subsonic speeds. Make no mistake, the F3D guys really are looking at this stuff today, because it suits the kind of contest and engines they are flying today... but up to this day, they're all running subsonic.
I'll go out on a limb here: but the first person to perfect a supersonic prop will dominate F3D for years (if the secret doesn't get out)