The stock MINI exhaust flows pretty well, but it's heavy. One simple way to add performance, tone, and lightness is to cut off the resonator (so-called "one ball" mod.) The JCW exhaust has excellent tone but is getting harder to find.  The Borla exhaust has a similar design as the JCW one and is available in two versions: sporty and loud.  If you want a lighter exhaust with good tone, try the Alta exhaust.