AP Rengar seems to me that it'd be like AP Sion, too reliant on one move for your damage. Of course there's the advantage of not having the shield broken, but eh. It seems like something that should be counterable.
It doesn't help that it's so rare to see a good Rengar. The 4v5 ranked game I just had was the one with the Rengar who ragequit. I guess it was my fault though, I shouldn't have said anything.
Anyway I personally had a decent score, and I went AP Eve again for the next game. I tried 9/21/0 going with scy's theory, since I guess you don't REALLY need the bit of extra AP, and executioner might be nice but you probably can get by without it. It seemed like a good combination. It was a long and grueling game with the AD carry complaining/harassing the support the whole time, but in the end Jarvan and I did a good bit of damage, I got a triple kill, barely missed quadra, went 11/1/10 in 40 minutes. I'm still liking AP Eve. If you can survive long enough to get deathfire and abyssal, it's fun times. Plus with the defense tree I get even more movespeed!