You won't be able to do that without root. If you do have root, check out Unicon in the Play Store. It'll apply any icon theme compatible with Nova, Apex, etc throughout your whole system.
OP: try the very latest version of either AOKP or CM 10.1. There was an actual issue with something on the system level eating up RAM. I was experiencing the same thing as you, and no matter what I tried, performance was abysmal. However, the latest builds of these ROMs have fixed the issue...
Re: Jelly Bean for Galaxy S3 end of August
Honestly, your best bet to keep root and update to JB is just to wait for a dev to push out a version of the updated TW JB ROM with root. Since you've removed some apps and modified your systemui.apk, trying to update to the OTA wouldn't be a good...