The shorter sidewalls on the tires for the larger rims account for the handling difference. Since they're shorter they flex less, so you get a more positive feel when cornering, much more precise feeling. But since they are stiffer they transmit more of the road imperfections. Typically things don't start to get really harsh until you get down to a 40 series or below, but that's just a ballpark recommendation as the sidewall height depends on series and width. 20's would have a really short sidewall to fit right on these cars, the biggest I'd go is 18's. And it will obviously also depend on how the car is sprung. A stock spring rate car would be much easier to take with a short sidewall than say my car, with 1.12" front torsion bars and XHD (soon to be super stock) rear springs.