I would stick with 13inch to be honest. It will allow you to run lower maybe. The cup cars managed with standard brakes.
Although, I dont think I would always want to run on slicks... you may find you have more fun on decent treaded tyres, or cut slicks. It will allow the car to move around alot more underneath - much more fun in my opinion. Slicks are more likely to wear suspension components and stress the gearbox too. You dont want to jump the kerbs with throttle and slicks on.
I have had brief experience with slicks and treaded tyres on track, and had alot more fun with decent road tyres - although you will want to use some worn ones, full tread is not good as the taller blocks move and squirm around - which doesnt feel nice.
Saying that, I will be collecting a heap of 13inch Michelin slicks soon. From the fronts of the Formula Renault's. Ill let you know when I have them