Race header creates no restriction in exhaust flow. A hi flow cat is still in the exhaust flowpath. So the second O2 sensor is reading differently than the first O2 sensor, so the computer sees the difference and believes the cat is doing its job.
Why not spend the extra fifty bucks and get the R/T cat? I'm getting it because of all the good reviews I've heard on here. It's stainless steel and I've heard flows better than the FC one. Go Here to see what I am talking about.