I would definitely not recommend having different tire brands/types front & rear. The different characteristics of the tires will make the car less predictable and less balanced during at the limit driving.
To avoid any drastic change in overall diameter (which will make your speedometer inaccurate) why not try the following:
Front 215/50/17 Dia. 25.5″ (OEM 25.9″) OK on a 6.5 rim width.
Rear 255/45/17 Dia. 26.1″ (OEM 26.6″) OK for an 8″ rim width.
Above based on BF Goodrich G-FORCE SPORT COMP-2
Diameters will change slightly depending on manufacturer and tire type, but should be close.