well if your going really small then the tyre wwould have to have a bigger sidewall. that naturally would mean more flex and less "stability"
but then again i don't think that that would be a big problem. as long as you don't use like 14 in rims =p
so overall i think if you use a decent size rim with sort of a low profile tyre, then it will give very good performance. the rim won't be too heavy and the sidewall won't be too big =]