Android 4.1.1 JRO03C built from source:
You can flash this right over 4.1 - just wipe cache and dalvik. GAPPS are included.
MIRRORS - thanks sashajovicic:
Only tested it very sparsely. Let me know what doesn't work!
EDIT: The following I have tested and are working
-Camera - has issues if you do a clean install, not if you "dirty" flash over v2
(looking into this)
-3G and wifi
-Custom 4.1 kernels work
-root (you may need to update the binary using SuperSU)
-all the included system apps
What doesn't work: Camera will have some issues on a clean install (full data wipe). Avoid that for the time being.
Fixed in 4.1.1: slight delay before launching apps