Thank you. I've read a lot of people go with 2.25-2.5. And i think I'll lose some low end torque 

Hello Chra,

Upgrading your exhaust to a larger diameter will improve flow and reduce backpressure which will result in more power, usually in the upper rpm range.  Most aftermarket exhausts go with 2.5" diameter.  Anything larger won't yield much additional benefits.  A more efficient exhaust will not damage your engine.  Check your local emissions laws to make sure you are within the rules.

As for warranty, you need to check with your local MINI service advisor since warranty coverage seems to vary from one MINI dealer to another.

Ken

i drive a 2012 base model Mini Cooper hatchback and I want to straight pipe it... what size should I go with? Can it damage my motor life I plan to go catback? and what performance will I lose/gain and will it void the warranty