Can I rollback the stock system by just installing an older image zip? I received the 4.2.x updates on my Nexus 7 and the performance has been awful.  I tried the Forever Gone hack and it helped but it's still much worse than 4.1.x.  I'm not so excited about doing a factory wipe in order to make my system almost as good as it was a month ago.  

I've never tried rolling back a stock system before, is it possible to download the official nexus image zip and install it from CWM recovery?  Or do I have to do a more involved wiping and flashing process?  My system is stock 4.2.1 rooted and unlocked, with custom recovery.
Don't use full image at recovery! It contains recovery too (CM can repair itself, but reflash of bootloader is not needed).

Use fastboot and flash partitions: system, data

You will lose all of your data on the tablet, but it is needed because of the different filesystem structure. Without reflashing data you will get a bootloop.