I swiched to 215/40/17 on my 97 850R, and aside from some minor rubbing, I have had no problems on the street, or on the autocross track. They look better (wider), and the ride feels better to me.

My favorite tire is the Yokohama Parada Spec II (215/40/17). They have a really aggressive look, extra big tread blocks on the outside edge (helpful when throwing your brick around the corners), and a rim bead to protect your pricey wheels. They are pretty soft, but that just gives them phenominal grip.

I have found the cheaper Kumhos (ECSTA 711) to wear poorly on the outside edges, but if you drive responibily they should last a long time.

I currently have Yokohama ES100s on the front, but they are noisy and have too much sidewall flex.

As a side note, I have been experimenting with hard tires on the rear and soft tires on the front in an attempt to induce a touch of oversteer to counteract the natural understeer. The handling seems more neutral to me, but it could all be in my head. The fringe benefit is cheap tires on the back that don't need to be replaced very often (just make sure the speed ratings are compatible).

Good luck,
Robert 850R
'97 850R, '96 850, '88 245