GAMI has done quite a bit of research into that, since their PRISM
system continually varies ignition timing with conditions (including
the fuel octane). Frankly, I paid more attention to the increased
cruise power at altitude than I did the fuel economy potential savings.

Check their web site and see if they have released any of their
numbers. [Since the existing magneto timing on a typical spamcan is
fixed, it's a compromise. The other side of the coin is that with
variable timing you also get a MUCH easier engine start.]