nowt wrong with running higher afr's, however peak torque is around 13.5 and peak horsepower is around 12.5.
running too lean for too long will make things get a little too hot for sure, too rich and you sacrifice power for safety.
basically you want it as lean as you can get without DET and EGT's getting too high.
the extra richness keeps the cylinders and pistons cooler, all engines pretty much have similar tolerances to heat and an rst is no different. i have run on boost at 12.5 without problems, but retarding ignition is needed to do this safely.
with careful monitoring you can get it as lean as you like safely, just start rich and lean out as it's much safer to do this.