The Ractive Header is a direct bolt on to the factory mid pipe containing the second cat. It is a 4-1. You can buy the mil eliminator for $21 from apexi and not worry about the second o2 and not have to do ANY mods at ALL!! Or..like myself, you can have a muffler shop weld in a second o2 bung behind the second cat ($10) and lengthen your second o2 sensor a couple of feet. No check engine light, and both o2's working as per factory.
4-1 headers are better for low end power, 4-2-1 headers are better for high end power...(I think, thats how it works on sport bikes so I assume the same for cars?...please correct me if Im wrong)
Header cost is $290 cdn...(+tx) Hope that helps.