Last I heard, Corbeau lost their FIA certification. Sounds like you need to get in touch with a scrutineer for whatever type of racing you're looking to do. I know I either run FIA-certified seats in the Galant or I don't rally. Period. I've never heard anything about supplemental cage structure to reinforce the seats.
Personally, I've had my eye on either Cobra or OMP from Subé Sports. Sparco/Momo are seriously overpriced. (Bonus: Subé clearly lists seats that are FIA-certified.)
This is a purchase I'm looking forward to making. <img src="/ubbthreads/images/graemlins/devil.gif" alt="" />