Ignoring paperwork issues for the moment: Is this an ignition system that
could be easily(?) retrofitted to a standard Cessna 172 / Piper Warrior?

Here's something to ease the paperwork issues:

http://www.unisonindustries.com/prod...ains_menu.html

This system varies the timing on certified aircraft engines,
but does not advance timing beyond the setting specified by the engine
manufacturere. I don't know if this is because there are no gains to be
had (25 degrees BTDC on an O-320, for example, should be plenty for
2700 RPM), or if the engine's TC would prohibit it. They do claim
better performance at any power setting below max, though, and smoother
operation.

Dan