On my wife's Cooper S I installed aftermarket steel wheels and Goodyear Ultra Grip tires (185 65 15). Lugs are torqued at 72 lbs as recommended in the manual. The cost was under $800. everything included.
There is a very slight loss in handling, a quieter ride and great traction in Northern Ontario's harsh winter.
Driving in a snow storm recently, I felt more secure with the traction of the Mini than that of my Quattro equiped with all-season tires especially braking and turning.
My wife tells me she feels quite safe in her MINI driving on snow covered roads. The traction control system works quite well.
Louis