Supposedly, A is rich, B is mid and C is lean. I have had varying responses between motors
The old SPFC corvette farted and ran like crap on A - super rich. Ran perfect on C
VSX160 early 2000s model, no difference, 2000s mustang no difference, 2000s ve160 medium difference (better rich probably because they were fairly lean in the last ve iteration)
Very rarely does it effect top no load speed, but it influences how it goes under load as you've found, generally A often is better
