Seems to me you can go up to a tire thats 2 inches wider then the wheel without any ill effects. So a 11-11.5 inch tires should fill it out nicely.
Now someone just needs to come along and tell me how wrong I am
On my solstice I have 18x8 and the tires itself is 10.3 inches across 255/40
The 914 has 9 inch wheels on the back with 10.8 inch wide tire 274/40