Just to set things straight. The 2.0 and 2.5 brakes are the same. However the GTA has larger disc's and fourpot calipers.
The point where the ABS kicks in, isn't as square as 147TS states. Modern ABS systems monitor the wheel speed up to 50 times per second, and when one wheel is slipping just a tad, this is registered, enabling the ABS to "kick in". this is a matter of programming. To my knowledge the ABS system on a 155 widebody and a 156 is the same (Bosch 5.5), except for the EBD (electronic brake distribution). The 155 has better stopping power, both cars being the same size and have the same tyres fitted IMHO.
I'm on a roll here, so while Im at it, I'll puncture another ABS myth. Stopping distance is the same with or without ABS on a dry surface. I have tried this once on an ABS seminar, measuring the brake force with a deceleration meter. What ABS is good for is, the abilty to steer and brake at the same time. As the tyres can only transfer a certain amount of force to the road surface, being steering force or braking force, ABS helps in distributing these forces.
In fact, when braking on gravel roads, it's best not to have ABS, as the gravel buildup in front of the tyres stops the car more rapidly, instead of the car stumbling on top of rolling pebbles.
Hope this helps