One of the issues with just lowering springs is that you compromise the function of the shock absorber as you have restricted its range of movement. You need to consider the change in camber, reduced suspension travel on normal roads so comfort is an issue. Drop links may require changing and your drive shafts (CV Joints) will be operating at an angle which they are not really designed for. If your going to use lowering springs, your also looking at possible spring seats and matching shocks at least. Coilover kits offer you a matching spring and shock set where the shocks operate to their full range of motion. Most Mini's have an average of 100mm or 4" of suspension travel which isn't much so a 30 mm drop is very significant. For car shows, yes they look nice lowered, for track days, my club has several drivers and non have lowered their cars, for the open public road, its honestly not worth it IMHO. For what its worth, my R50 is on coilovers, Car was lowered for a car show and after was reset to the default ride height and as such, drives very well, when it was lowered, the ride was harsh and often bottomed out on roads and annoyed the hell out of me and that was with a 25mm drop !