I tried CAS 3 and it worked, but only up to ~700MHz at ~2.2V. I didn't mind trying higher as I already killed lots of sticks with voltage.
I didn't try too high frequencies either as tighter timings seemed to be faster for my setup.
Btw., the correct sub-timings gave me the same boost in performance as switching from 800MHz 5-5-5 2T to 900MHz 4-4-4 1T. Quite noticable at least (~5% gain in SuperPi)