i agree with not attacking buffs from the belly, especially the b17...
i fly the 17 alot and besides a straight and level 6 attack,, the belly attack is the easiest to deal with.
when attacking from above, it is much easier to jink or go knife edge (which provides a much smaller target.) however the belly atack is usually performed with your wings // to the horizon,, which gives the buff gunner full view of your wing tips and horizontal stab.
this disadvatange is in addition to the fact that unless you have tons of speed, you are going to stall out within spitting distance of the buff guns.