Dunno, after a lot of trying with masteries and runes, I reached a decent setup. I can get to level 4 in minute 4 without any leash at all (lots faster if I get it, ofc). Actually I know that's probably not even close to the best of times, but I manage to do well in the first ganks and such, even getting a bit fed if lane partner can't take the kill. Problem is I start to fall behind when the game goes on (although most of the times, my lane partners had a fair share in that :P).
Talking about normal games. Not saying Rengar is great nowadays, but I don't find him SO bad and I think he has potential (if he wasn't so overnerfed)