From my experience i could only get 195/50/15 on 15 X 6 rims without doing mods to the guards. Although i use adjustable castor rods, so that i can move the geometry around a bit.
i have since fitted 195/55/15 Falken Azenis street legal slicks on 6 inch wide rims. I cut the lower corner / edge of the guards.
Very soft tyre, great grip, dont last very long though ( it doesnt bother me as it is only driven every other sunday)
Chech out this article, it compares yokies, bridgestones and Falken Azenis.
http://www.autospeed.com/A_0593/P_4/article.htmlCheers