15mm spacers are actually quite a bit. That should give you inboard room for 30mm (ish) wider tires on an otherwise identical wheel
The 15s really help flush the sidewall with the wings, and help a bit with that weird perspective created by the raised suspension and lengthened plastic arches. The car definitely feels more planted and neutral with the extra 30 mm of track, but I don't have lap times to back up that notion.
My summer setup is 215/45 on 17x7 ET48 (equivalent to a 6mm spacer) and looks like I've got maybe 10mm of room to the strut up front.
Added a couple of pics. It's noticeable to me but no way anyone else will see the difference. I parked next to another mini other day, not a SE, and difference is pretty slight in my eyes.