| Nice Elan BTW Swamos.
Unsprung weight as far as performance cars go is an absolute killer. The theory here has been gone over previously so I won't go into that, as I've said on a previous thread I've a PRB Clubman that I sprint and I fitted lightweight 3 piece 13" rims with really lightweight road legal Formula Ford tyres replacing cast 14" rims with R spec rubber. Difference, massive on the track, definately accelerates faster, turns in better etc etc.
On a road based Mini, forget about it. I'd argue the case if you were fitting 19" rims over 16" but between 16/17" factory rims?? get what you think looks better (I'm going 17"), I would be interested though in the weight of R90's to the 17" S-spoke rims though.
As far as damaging a rim on 17's compared to 16's with run-flats you won't have a problem as the sidewall is quite thick compared to a normal tyre. | 
