I ended up having 245/40/17 on the front and 245/45/17 on the rear at one stage(had to replace rear tyres due to punture).
Every bump I hit caused the front tyres to get drive. Now my ATTESSA needs a good bleed (have same size all round, works fine when it doesn't crap out).
Run the same size tyres all round. You will risk wearing the ATTESSA out. Note since your rear tyres will be spinning quicker than the front, I'm guessing the car will be sending torque to the front wheels a lot more than what I experienced.