Sorry to crash this post, but I read somewhere that if you fit a sports cat (turbo back) the only way to "turn off" the CEL is to go to stage 2. Is that correct?
There's a way of doing it mechanically rather than via software. Involves fitting a spacer to the lambda sensor or something, but from everything I've read it's far from a guarantee that it'll work.
I don't see any reason why someone with the knowhow couldn't code out the sensor and retain whatever map the car is running on, but it seems a bit pointless when a Stage 2 map does the same thing as well as optimising the map to the new hardware. And it's not just power where you'd benefit, the afr and other parameters will be properly set up.