I just saw this thread and wanted to add my findings. Although I messed with this years ago, my finding was that although a leaner afr produced more power, it wasn't enough to make me want to run a lean afr, even on race fuel.
The car was a n/a mustang gt. No ecu just straight carb for afr and distributor controlling advance. I started at 13.0, 12 and 11 afr. The power difference was slightly less than 10 hp going from 13 to 11 afr. That's with no timing added or subtracted due to timing controlled by rpm's and the distributor.
On same car same day I ran 12 afr and added 4* advance to the distributor and picked up close to 40hp at the wheels. So timing plays a lot larger role in producing hp than afr ever would.
To me its not worth tuning to a lean afr to produce power, go rich and find a good balance between boost and timing to make power on whatever octane fuel your running.