If I consider that you have the "basic" GTD with granada original setup, 10x15 rear should be considered as a maximum IMHO.
I've had some 295x15 Avon some time ago on 10x15 wheels, but finally downsized to 275.
Altough something smaller will provide a better look, IMHO again, with the wheels more inside the wheel archs (something around 9x15), more "authentic". Your own appreciation and the look you want to achieve will dictate. Check the offset carefully, and make some mockup (what I didn't, and now regret !)