AOSP based roms do not have hardware VSYNC enabled yet, which is the largest component of Project Butter, which is the most wanted feature from JellyBean

. There are other performance tweaks however that are enabled, and there is a pretty noticeable performance edge to AOSP vs Sense already. There are issues still cropping up and disappearing and then coming back though, as the developer tries to get all this new code working on a device that he has no real drivers for. There has been a rough patch this last month with bugs though, from screeches on recorded video, to Nav crashing, to GPS not working, etc. It is very random though. Not everyone sees the problems. Best to try it out and see. Sounds like today's nightly got rid of the video screech, which is a plus!
I would start at CM 10 for a baseline on the AOSP side, but they are based off the same codebase, so any will suffice for comparison. I am running on Viper XL for the last few weeks. It also has quirks, but it has the CM toggles in the notification, BT audio works in my car, and it runs as good as any Sense I have seen. There are almost too many options though- I nearly gave up on it at first as being too much (All the Venom tweaks, Venom Hub, and settings mods. Lots of stuff there). I used to use the old CleanROM dev as my Sense ROM, but Viper is that spot now. I am still watching the changelogs on CM and test once a week or so to see if my particular gripes are better.