When, if ever, would want to upgrade your master cylinder and power booster for your car? Larger calipers? Rotors? Is there a different system in the Cobras than the GTs since they have different size brakes?
I upgraded my brakes last year to the bullit in front and cobra in back. The shop told me there was no need to upgrade my booster or master. The only thing I wished I had upgraded were the pads. Cheap pads make a noise. Other than that the car stoppes fine.
Hope this helps
Brakes are nothing more than a hydraulic system. Hydraulics work off of the equation:
F1/A1 = F2/A2
where
F1 = input force
A1 = input area
F2 = output force
A2 = output area
By swapping calipers, you're changing the size (and usually number) of pistons in the calipers. This changes the amount of input force (F1) required to elicit the same output force (F2). By changing the master cylinder, you are changing the input area (A1). This can alter the input force required resulting in a softer or harder pedal feel.
So, to answer your question, you would want to change the m/c whenever you feel that the amount of pedal force required to brake is unacceptable (the pedal is too soft or hard).
Brakes are nothing more than a hydraulic system. Hydraulics work off of the equation:
F1/A1 = F2/A2
where
F1 = input force
A1 = input area
F2 = output force
A2 = output area
By swapping calipers, you're changing the size (and usually number) of pistons in the calipers. This changes the amount of input force (F1) required to elicit the same output force (F2). By changing the master cylinder, you are changing the input area (A1). This can alter the input force required resulting in a softer or harder pedal feel.
So, to answer your question, you would want to change the m/c whenever you feel that the amount of pedal force required to brake is unacceptable (the pedal is too soft or hard).
For the most part, the booster (or vacuum canister) would remain the same. All it does is maintain an area of low pressure (vacuum) to assist in applying pressure to the system. As long as you aren't running a cam with insane numbers (and therefore ruining your ability to build vacuum), the stock booster is fine. And usually the guys who are running cams like that are running manual brakes anyways.
If the brakes are too stiff, you'd still change the master cylinder. The stock GT m/c uses a 1 1/16" bore whereas the Cobra uses a 15/16" bore. Swapping to a Cobra m/c will reduce the required pedal force to result in X output force by 23%.