M3 headers on non m3 power difference

I'm putting in the m3 exhaust pipes and the guy I'm buying off is selling the headers as well. I've seen mods to change the header flanges from 6 piece to 2 piece and most likely im gonna do it to stay CA legal but I'm wondering if anyone knows the power difference between m3 headers and catless headers? The m3 headers are much less restrictive but they still have the cats. Instead of what a 15hp bump with catless it's gonna be like 8-10?