One of the easiest weight-saving mods you can make to your MINI is to replace the stock battery with a lighter weight one.  Lithium Ion batteries are ridiculously expensive, but Absorbent Glass Mat (AGM) are not and they weigh about half of stock.  For this application we used the Braille B2618 which an 18 lb., right side positive sealed battery.  It has 472 cold cranking amps and is rated for 26 amp/hr. Braille also makes a very slick aluminum cage to mount it that fits nicely in the MINI battery compartment (with some slight modifications). The neat thing about sealed batteries is that the can be fitted wherever you want and mounted vertically or horizontally.  If we ever corner balance the car, we’ll think about relocating it, but it works fine now where it it.