IIRC, two or less levels below you, you get full credit. 3 to 5 the exp starts dropping but not all that much really. I have done some grinding with the zombies (L48) at the Vile Lab with my VR14 and was getting a decent amount of Exp with each one. I even have an addon that shows me the exp per kill and it does not seem like it really matters. I also had a character that was constantly 5 to 6 VR ranks over the mobs (started Gold at VR10) and it did not seem to impede leveling progress. In fact, she reached VR16 before really doing much of the last Gold zone.

So, it's still worthwhile to fight mobs even if your over leveled. If your looking to maximize the amount of Exp you get per kill, then take on mobs that are on level or one to two below you.

Finally, Orsinium Mobs are always on level as far as Exp goes, no matter what level you are.