The easiest way to increase brake torque (stopping power) is to run a higher CoF compound pad, as suggested above. The easiest way to add brake capacity would be to increase rotor size, diameter and/or thickness. Given the application, you could opt for Skip's suggestion to swap over to a 12" nominal GM tall spindle and inherit the bigger brakes along with it, however given your location of Japan, that's probably not going to be as simple as heading down to a local wrecking yard.

What's the intended use for the car?

Tobin
KORE3