I know there is the 15" haters and lovers on here but IMHO 15" would just ruin what the car is all about essentially, chuck about good handling, think about it, slower acceleration, reduced suspension travel, tramlinning due to wide tyres, expensive tyre bills, more chance or killing tyres/alloys on bumps cos of the low side profile.
So it's fair to say from my POV 15"s would be the worst thing you could do to to Cinq, I can even feel the difference going to 14's, if it wasn't for the GT Brakes I would be back to 13's like a flash.
Liam