saltshaker Jon it depends on who make of car it is. I am not going to answer for any one but from ERA experience. When I got my car I started with the 11" rotors in the front and the car stopped well. There was a bigger improvement when I went to the 12" rotors and dual piston setup. The car really stop.
The car doesn't have power assist brakes and IMO doesn't need them. ERA sells and outboard brake setup for the Jag rearends. These brakes may also be dual piston in the back. If not for me wearing my 5 point harness, I would have put my self through the windshield more than once. 60-0 mph my car stops in 116 ft with no lock up of the front wheels. This is quick. If your car has the ability for ABS and or Traction Control, IMO it is worth the added money to have the systems installed. Safety is always the most important thing.
I have driven for the first 4 years my CJ5 jeeps that had neither PS or Disc brakes. 4 wheel drums 11"x 2". For a car wieghting 2,500 lbs also and running l60x15" tires, this taught me how to drive without power anything.
A manual brake master cylinder, depending on bore size and ratio of pedal to cylinder puts out about 1,400- 1,600 pounds on pressure to the calipers. With power brakes you are going to about 1,800 psi to the cylinders and calipers. If you want to get crazy, a hydro boost system is in the 2,200-2,400 lbs. Bigger trucks run this system. They are also selling it for all cars now and it works on the land barges real good. It uses the P/S pump for power instead of vacuum from the motor. They are working on electric brakes for cars in the next couple of years and possible a magnetic system after this one. As for power brakes on a cobra, with a good 11" front rotor setup for street driving NO. For racetrack driving, bigger brakes and maybe. Flushing the brake system at the track needed to be done more offen, because you get a softer pedal and the fluid is getting boiled. This is RESEARCH you will need to do before buying your cobra.
Rick L.