What it comes down to on average, is AEG's just don't fluctuate very much in terms of muzzle energy from heavy to light ammo.
GBBRs however, do. And by a lot.
So (at least in Mb), we've made it mandatory to chrono GBBRs with the weight they'll be using in field. From that it's naturally an easy step to do the same with AEGs.
On our chrono's we've got a list of what BB yields what fps at 1.6J, so we're always determining muzzle energy.
-you're definitely not playing with the hop OFF
-some guns, like bolt action rifles and GBBRs, actually GAIN fps by applying proper hop pressure
-some guns yield minimal fps with the hop off. Sometimes the air nozzle will push a BB past the hop rubber if it's off and it'll roll halfway down the barrel before it's fired
-some guns, AEG's included, can gain muzzle energy by switching to heavier rounds
So chrono with the hopup set properly, and ideally with the weight of ammo you'll be using in the field