As we have established in the CIU weapon balance program - Part 4: Photon Swarm - #45 by 1galbatorix1 thread, the bombers are pretty much broken, and I don’t mean that as “OP”.
As it is right now, any runs that include either Neutron Gun, Laser Cannon, Lightning Fryer or Riddler are just fine. In fact, it’d probably be even more of a challenge to complete a mission with a bomber than with any fighter if you’re using these weapons.
As for the actual power increases, the BX-6 has more power levels (there’s no gap between 
10 and 20 as far as I know), and its max level is 23. I assume that the heavier bombers have higher max levels.