215 would be the max on a 6" rim. What's going to happen is your going to get a lot of foward and braking grip but what's going to end up happening in a hard corner is the tire will roll over, you can combat this with some extra tire presure but where its going to really bite you is in hard left right transitions.

205 may even be too big you probably may want to look at a 195 since the rx7 isn't a huge torque monster and the brakes aren't huge