Ran 30/-4 (26 caster) last night, and the car felt really good! It was a bit tricky making sure I had my caster inserts in backwards, correctly
Qualed 2nd, and finished 1st with it. Car was very very good. There is a video of the run out there somewhere.
I think 30 / -4 gives great steering in the infield, with great off-throttle rotation.