I just had a trackday and took my Clubman Cooper S for a spin. Of Course, had some modifications, JWC brakes, better coilovers, strutbar, rear brace, upgraded to 230 whp with bigger intercooler etc. And I kicked similar R55's asses :-)

My findings: Difference between R55 and R56 are very, very minor on a track. And my guess is the R55 can have an edge.

I was wondering if there are soms Mini's out there that have the full underside of the car 'flattened' for ground effect . I met some Lotus owners who experimented and got great results. And in the Prius hyper miling it is common practice.

But usually there is a wind tunnel and a lot of aerodynamic science stuff involved because you can get dangerous vibrations and front-to-back rocking of the car on high speed.

The ground effect is interesting because the mini is Absolute King in the tight slow bends, but lacks in the long high speed turns because it is sort of a cute little brick, and these kinds of turns you see the advantage of rearwheel & spoilered cars.

But perhaps I overlooked something. Bolting a big piece of plastic to the underside should not be a big problem

 

Heat, weight, air to cool those hot things.

It is popular to open up the rear facia in some JDM cars as it acts like a big air damn sticking down. A lower front splitter and side skirts will keep the air out from there to start with. This is "ground effect", which will increase grip, but you will loose straight line top end. Nothing new in that book!