You can calculate brake torque if you know the deceleration of the car, the contribution of aero drag and the mass of the car.
E.g. say a peak deceleration of 5g is achieved, of which 1g comes from aerodynamic drag. This means 4g is coming from the brakes (including ERS). Assuming a car mass of 700 kg gives you a braking force of about 27.5 kN. Multiply by tyre radius (0.33m) to obtain torque of about 9000 Nm for all wheels combined. At high speed the brake balance is perhaps 60/40 so the rear axle would be generating about 3600 Nm of braking torque.
With or without ERS, the brake torque has to be delivered by the tyres. Most of the braking action is limited by tyre grip. Only at high speed enough downforce is available so that the tyres actually have more grip than the brakes can handle. In these conditions, the brakes are limited by the stiffness of the system. The regulations stipulate that the brake callipers must be made from aluminium. Additionally the hydraulic lines and other components will show some flexibility. The driver can press the brake pedal all he wants, but at a certain point most of the extra pressure he delivers will be lost in deformations of the brake system rather than exerting more force on the brake pads.
When you get below a certain speed, the available downforce decreases and the deceleration of the car is limited by the tyres. In these conditions, having ERS does not make you brake any harder. With high downforce available, ERS may actually help slow the car quicker. It would be interesting to see if peak deceleration has increased since we have KERS.
There's an interesting topic on high speed braking here but it's from 2010 (when teams chose not to use KERS):
http://www.f1technical.net/forum/viewto ... f=6&t=9042
