I'm pretty sure you always need to tune for headers to get the best performance.
As I understand it, the issue with the CEL / emissions / O2's is that most "high flow" cats don't work very well, so the rear O2's register that they are not working and then the rear O2 has to be turned off to avoid that code / CEL. But that's a catch-22 because you cannot pass emissions with the O2's turned off, all sensors must be reading "ready"
Yeah, that was my worry since we have OBDII/emissions testing here. I talked with the tuner today, and he assures me that the car will pass all testing after the install. Hopefully he's a straight shooter here. He's not pushing a sale, and busier than a 3 legged man in a butt kicking contest. https://www.facebook.com/ProDynoSC
Well, I guess I'll have to trust this and if it doesn't work out so be it.