In order to counter steer you need the rear tires to turn a minimum of half a rotation more than the front. So for every one rotation of the front the rear does one and one half. You cannot achieve that much rotation with a larger diameter tire. I saw an exceed madspeed with 2.0 counter using a team associated gear.
With 1.5 CS ratio, don't worry too much about steering angle, see if you can shave off the steer stops on the steer knuckles this should give you the angle you will need.
this is the link
http://www.youtube.com/watch?v=qO5I7_vlszQ