It's amazing isn't it? You give some people, usually dealer techs, a bit of technical knowledge and they seem to think that BS always baffles brains. I find it insulting that some of them think their customers no absolutely nothing technical about their cars.
Anyway your wheel and tyre combo seems ok to me. Did you not ask why the Cooper S comes with 16" wheels as standard and why it was optimised to run on that size of wheel? I wouldn't have been able to help myself
There are a lot of reports of worn/replaced bushes on here, so Mr Crofts may well be right. Doubt it's the wheels and tyres though, unless your pressures are all over the place.