I'd love to see one of your pie charts on this but I realise its not possible in the same way as fighter usage. in terms of balance all I mean is that the big 3 must do 90% of all buff damage in the MAs, if that was fighters they would be perked. I imagine the effect of no perked fighters would be similar (eg. 262/Temp/C-hogs getting 90% of all fighter kills).
There are not that many buffs to choose from as we do have fighters. And buffs are competing with / fighting against those. You can't just take an isolated look at them. The bomber has to get to the target & back, and it's 190A8's, Ponies, N1K, Typhoons that are hunting him.
We have the big three: B-17, B-24-Lancaster
Only one really capable medium bomber: The B-26
Four more medium bombers, that have many drawbacks in LW, even the very fast and quick climbing Ki-67 has the problem of having a light payload. Flying a heavy 110 is often much more effective than a Ki-67 flown correctly to it's strenghts
And finally the perked Ar 234.
Again, heavy bombers are not unbalancing. If you just perk them if it's too dam many of them compared to the B-25C or Boston, you have to perk the P-51D, N1K, Spit 16 too, because it's too dam many of them compared to the 109F4 and other "light" birds. Or look at the CV segment: Almost everything that's rolling from a CV deck with bombs is a F4U-D or F6F. Perk them too, let players earn their right to fly those by flying TBM's & SBD's
If you just want to encourge more diversity, you will have to find another approach. One that is fair to all sides.