I don't think that the rolling radius is that vital, you can have your speedo recalibrated if needed (and yes, it must "be fitted and work" (not calibrated) for an MOT) - the look is much more important IMO?
The pic above is a perfect example - it's a totally different look to the original MX5 and a totally different look again to the original 250 - which probably would have been 80 or 82 profile on 15" or 16" rims (depending on year) - it's all about the rubber to wheel ratio
