In the Mini's if you run super light wheels the steering is very responsive & "razor" like but bumps & irregular road surfaces can produce jarring & wheel bending results. Fuel economy may be improved a bit but tire resistance is a more critical variable. In a car like the HCH I would guess the typical 22-25lb OE wheels is just about ideal for the suspension & battery regenerating system. IMO 15"s would be about perfect on the HCH especially given the very over-boosted electric steering rack: One would not want hypereactive or jumpy steering reactions.