The stock class autocross guys must stay with stock width rims under the rules, but they can stuff as much tire as they want. The result of stuffing wider and wider tires without giving support to the tires with wider rims (again, because that's the rules in 'stock/street' class), is that we can get more ultimate grip but steering feel does take a nose dive. Watch some of the youtube videos of the oversize tires sloshing around on rims when the ratio of rim to tire is poor and see what you get. Autocrossers make up for the slop as best they can by jacking the PSI much higher, but there is only so much you can do when the tire sidewalls are so far from straight.
I like steering feel, so I go the other direction.....tending to use 10 inch rims for the same amount of tire other guys use a 9 inch rim. So, for example, on track days I have a 255/40 tire on 17x10 despite fact most MX5 guys would put that tire on 9 inch wide wheels,...and I would use 11 inch wide if I could find it! The result of 10 inch with that 255m is that my tire lasts longer and turn-in response is amazing. Look around at the NA/NB track guys and notice more and more put a 205mm tire on 8 inch wheels, and 225mm tires on 9 inch wide wheels....the reason is because it is faster, and MUCH more confidence inspiring to drive at the limit when the tire sidewall is stabilized. Since that tire is NOT sloshing all over the rim you can actually feel what it is doing!
Every choice is a tradeoff. The upside of giving the tire proper support is quick and precise steering and longer tire life and more grip from a given size. The dowinside for stabilizing the tire sidewall includes stiffer ride, more risk of curb rash, etc.