before 4.11, i remember I had to keep fuel load around 30% and sometime add a little flap and keep engine cool. it seems to work okay. i wonder if those guns are heavier than full bomb load? maybe this is historically accurate for the plane to be underpowered with that setup. but I only guess.
edit:
interesting stuff here. these guns were heavy. the d and g series were both upgraded to 211-j for more hp. this increased bomb capacity. but also consider that addition of more armor protection to these models probably added weight, so offset some of the benefit of the hp upgrade.
http://en.wikipedia.org/wiki/BK_37
top attack
http://en.wikipedia.org/wiki/Top_attack
I never tried top attack on the heavy tanks in the game. maybe that is the trick?