Right now only "heavy" buffs are usually upped because only the B17/Lancs have the bombload and defensive ability to actually achieve something in the game. B26s are usually taken for their speed and the 50 cals. But ive yet to see Ju88s, Ki67's, Bostons, etc. , aka, "light buffs" used in the same regularity.
So here's my suggestion:
The lighter the buff bombload and/or the lower the buff's survivability (aka defensive guns or speed or both), the more buffs per formation.
Aka:
B17 (best defensive ability) : 3 buff formation
Lancaster: 4 buff formation (crappy defense but good bombload)
B26: 5 Buff formation
Ki67: 5 buff formation
Ju88: 6 buff formation (excellent bombload but very low survivability. SLOW to boot)
Boston: 8 buff formation (crappy bombload, weak, minimal to no defenses)
The above would allow players to have just about the same impact on a target if they reach it (more buffs in formation =more bombs + more chances to get to target with at least 1 buff) regardless of what they fly.