That is what I did for AutoX--255/40/17 Hankook R-S3 on 8.5" rims. They don't look too wide, but I haven't driven them in any events yet so can't say. I'm signed up for the BMW CCA AutoX this Saturday at Qualcomm, so maybe my new tires and I will see you there? (I don't do SCCA yet.)
I've seen this question come up a few times in the track, AutoX and club racing forum on bf.c. Half the respondents say what BartenderPlease did--Do It! The other half say 255 width tires feel more precise and grip better on a 9" rim and you should not run more than 245 width tires on an 8.5" rim.