First, the subject begs the question- why are you looping over and over, and unless you're in a P-38, why are you doing it?
The P38 loves to loop, and absolutely adores any single plane that'll try to follow it. With its legendary low speed stability it can absolutely crush the highest performing a/c in the game. While they're tyring to keep their a/c from spinning and augerin', the 38 will pop over neat as you please and get a kill.
Loops have their place. However, 9 times out of 10, you're going to be coming out of it with *less* altitude then when you started.
I think what you need to consider is slicebacks, immelmans, and cubans. All are more beneficial e-wise, and they all serve the critical purpose of changing direction with an e-balance. The loop fails to change direction, so unless it performs spectacularly, there's no real reason to do it.
Back to having their place- if you're in a plane with decent smash, against a poor vertical performing opponent, it can be the king of the finishing moves..... if the other guy bites it hook line and sinker- a classic loop-rope is a beautiful thing.