[g.rz]

I recently had the same predicament, got the headers first and the OFT a few days down the line. I recommend getting the OFT first if you really had to pick, obviously both at the same time is ideal, however if you'd rather hear your exhaust note/sound change and receive little to no performance gains what so ever get your headers first.

Getting headers first from my experience didn't cause any issues, other than getting a CEL after around 30 or so miles until I got my OFT to tune my car. Get the OFT first if you want to see performance gains, although for me the changes didn't feel too huge until I did the Stage 2+ E85 tune.

Going cat less on your headers along with your current setup will definitely make your pops louder, with an OFT you can also adjust timings to get pops throughout all the gears at a certain RPM range by feathering your throttle (from what I've read it's not ideal for your exhaust manifold and the engine in general but I just sent it)