Silvia Brake Bias (S13 and S15 Rear Caliper Piston Size)

In the search for greater rear end brake bias: I had been informed that S15 rear calipers had bigger pistons than S13.

After purchasing a set of S15 rear calipers I got home to measure up:

38mm Piston Diameter on the S15 AND S13 rear calipers

As captioned, I found the S15 and S13 rear calipers to have the same piston size. Despite nearly everyone on Nissan Road Racing who believe that S15's have larger rear pistons - it turns out that it is nothing but a myth...

Now to figure out another way to increase rear bias... Master Cylinder?