Looking at the math and the Air combat table:
A heavy bomber with a A2A value of 4 vs. a Me109 with an A2A value of 5:
Me 109 kills the bomber on a 3, returns it on a 6 or less.
Bomber kills the Me109 on a 2, returns it on a 5 or less.
If we make the Hy. bomber a 3, but attacks against it are a +1 then the results are:
Me 109 kills the bomber on a 3, returns it on a 5 or less.
Bomber kills the Me 109 on a 2, returns it on a 4 or less.
So actually, I think this suggestion makes it actually worse for defending interceptors. So maybe not a good idea after all.
I like the 3 or less kill, 6 or less return results for an Me109 against the hy bomber - the +1 column. You get a good mix of returns on that column which represents damaged bombers going home.
But, I like the bombers shooting back on the -2 column (2 or less kill, 4 or less return).
So - maybe the right adjustment is make Hy bombers a 3 Air to air value when firing A2A, but a 4 value when defending A2A. You actually don't gain much over the 4 A2A value for Hy bombers though, one lower return result for defending fighters - which does not mean much.
If you make them just a 3 A2A value with no modifiers then results are:
Me109 kills the bombers on a 4, returns them on a 6.
Hy. Bomber kills the Me109 on a 2, returns it on a 4.
With this, you kill bombers a bit more, but you do not increase the suvivability of your interceptors at all - so does not help your problem either.