I don't think big guns are a problem. All this big guns we hear people talk about are really heavy and were installed in big heavy twin engine planes.
This are not the planes to wipe out the arena. They are still big heavy sluggish sitting ducks against single engine fighters.
Just looking at a photo of those guns you realize those were big guns. Big guns have really low rate of fire and kick like a mule. You'll probably have to aim again after every shot.
The 75mm cannon on the B-25 was manually loaded so it will have the rate of fire of a panzer.
Bk5 is a 50mm cannon with high muzzle velocity that will have powerful recoil. The Me-410 in itself is not the best plane to wonder around agile single engine fighters even without the huge cannon on board.
This planes won't be deadly enough to mess up the balance of the game, but a lot of people could have a lot of fun playing around with them.
If i'm in the gunsight of a B25 in shooting range he can kill me just as easy with the eight fifty cals, or the Me-410 could just as easy blow me to pieces with six 20mm cannons or 2x20mm+2x30mm cannons.