Try about 33 psi front and rear cold for your tires and adjust from there.
Not too firm riding but enough pressure not to overwear the treads.

Almost any non runflat tire will run more comfortable than runflats

Grid is OK tire but depending on what tire size you need

try to check out the Continental ExtremeContact DWS if you need a longer wearing all season tire that will do good in the cold or snow

Or the Continental ExtremeContact DW if you want a good Summer tire for relatively lower cost.

215/45-17 is OK tire size though a bit taller than OEM tire diameter and should work fine for any MINI with stock suspension. If you lower the MINI then there is potential to rub a little on the rear wheel arch plastic but you can trim it and adjust alignment to get more negative rear camber- just don't do too much as this may increase understeer.

 

I would steer clear from the Grids, these were on my car when I bought it and it was so loud that I thought a wheel bearing had gone out. I then found that the rear tires were cupped and wore really badly. I looked at the reviews from tire rack, 80% of them mention how loud the tire gets after 10K. Seems like everyone loves them right away but then hate them and replace them before they are worn out because they just can't stand the noise. Also I wasn't impressed with them on snow either...

 

The tires that I bought had like 200 miles on them - they are basically brand new. I'll sticking with them until they wear out or become a problem. I was just looking for advise on what tire pressure others have used with success. Here in Atlanta, we have occasional snow and some ice and these are going to be better than summer tires which is about all I need.

 

Been running grids for a year now with no noise and snow traction has been great as well. I have seen the reviews but we don't see it and grids are on both cars.

 

Grids here too, they been on for 1.5yrs. no issues, in fact they did great in the snow we had the Winter of 2009!!