The theory is during part throttle cruising you can lean the mixture out a little more if you give your motor some more advance. It just doesn't seem to work on performace versions of B20's. I put this down to great atomization from SU's or injection combined with an an intake port that provides reasonable swirl into the cylinder.

Vacuum advance would be useful with boosted engines. You could cut back the timing provided with centrifugal advance during boosted suitations & then use vacuum advance to bring the total timing up to normal during part throttle cruising.

If you want some serious gains in MPG, get one of John Parkers street ported F heads & a free flowing exhaust systems. Best bang for the buck would be his $625 head.(You provide a good F head, he ports it & fits dual springs).