I don't think we should leave out historical loadouts just because they might be unbalancing. Thats the entire reason for perks. Its the same think as perking an unbalancing vehicle, just like the Tiger II.
If people want to pay, say 5 perks, to carry 2x 2000lb bombs on their P-38, I see no problem with that.
And P-38s are (in my opinion) easier to deal with, especially when flown by average sticks. Not a whole lot of people are real adept at using the P-38 to its strenghts, while the P-51 is much easier to do that with. Just up something fast, and you'll either force them to drop ords, or kill a lot of P-38s.
I rarely flew the P-51 after I got into Luft aircraft and GV's, and I know some who would only call me an average stick. Check my sortie stats if you care to (49Jager, or Jager for the most relevent time period). But after maybe 6 months without using the P-51 as anything but a bomb-truck, I took it out to a furball and promptly landed 6 kills with it.