I have an 07 56s wondering if the CEL from going catless does anything to decrease performance? Like cut timing, fuel, etc etc. Or is the only downfall the dash light

 

it's just a dash light. Not likely to cut timing or fuel as long as your sensors are all working just fine. The CEL stems from the post cat O2 sensor saying that the the catalytic converter efficiency is not normal anymore (of course if it's completely absent).

 

Yup, what he said ^^
The Light is just annoying. That’s about all. A tune will get rid of it.
You could try one of those bung extensions that are supposed to fool the 02 sensor. But they rarely seem to work on Mini’s. I had to wait for about a month and a half for my tune after turbo and downpipe were changed. So I tried one out.....didn’t even work for a few seconds, lol. But some people say they have had some success with them.

 

There is also the CANTool option to get rid of the CEL

 

I have just put one in classifieds for sale $175