This is a first! I have never seen a driver looking for less contact patch in track tires. If you are running a DE, balance is more important than lap times and your brakes will last a lot longer.
225 on the rear will help balance the car at the expense of a little lateral grip and stopping power. Dunlop says you can mount a 225 on your stock 8.5 wheel:
http://www.tirerack.com/tires/tires.jsp?tireMake=Dunlop&tireModel=Direzza+Sport+Z1+Star+Spec&partnum=245WR7Z1SS&vehicleSearch=false&fromCompare1=yes
Have fun!