Any bomber is easier to kill at 15k than 3k. Bombers hugging the deck make high angle, high speed attacks very dangerous for the attacker.
Quite the opposite for me, Buffs are harder at 15k then 3k to shoot down. Its far easier on the deck as Buffs are SLOWER, with no room to work, a 262 for example can easily get the speed to attack any which direction, above 15k things start to slim out.