I will add that I have the Porterfield pads on two cars.
The pads don't necessarily add friction to the system to give you harder stops.
What they do is give you less brake fade at hard use like in hard autocrossing.
In order to get more braking you need a larger brake system. Bigger pads. Calipers with more clamping. Larger diameter rotors for greater braking leverage.
In a Pantera, although it isn't particularly difficult to improve the brakes, it is a little touchy.
Mid engine cars are quite a bit different then front engine cars are as far as weight transfer in braking. Any big changes are going to require rebalancing the system front to rear and you just can't do that on the street normally. You need to do high speed panic stops and that really needs to be done on a closed course.
The largest danger is putting too much braking in the rear.
Look at how small the original rear pads and calipers are. Those are a little overkill but that was done so that the rear of the car won't come around on you in a panic stop.
Any improvements in the rear are necessitating an in line adjustable proportioning valve.
To show you how delicate that can be, some race cars have that adjustable valve in the cockpit within reach of the driver so it can be adjusted in real time.
As far as the strength of the vacuum goes, the Pantera brake booster adds around 150psi to the brake pressure going to the calipers.
It SEEMS to be designed for 22 inches of vacuum. You will only get close to that with a stock cam and CJ engine. If you have for instance, 15 inches at idle, you are getting less boost to the brakes.
When you go to a full manual master cylinder you need to increase the size of the master cylinders bore to increase the pressure. There you will feel it in the brake pedal effort.
The stock Pantera master bore is 24mm. That is about 15/16" or the size of a stock Mustang master with power brakes. To go to manual brakes with about the same braking pressure, you are 1-1/16" to 1-1/8" bore. That's like stepping on concrete on a street car but probably to be expected on a race car since very little brake pedal movement would be necessary, pressuming your leg is strong enough to move it.
I've got a Compcams vacuum pump in the car. The thing is noisy but with the noise of my solid lifter engine, 180° headers and Webers, it gets lost in the engine racket and it is all behind you killing innocent bystanders while you play with the tweeters on your stereo system in the cabin.
With the pump, my brakes are getting 22 inches of boost. That's maximum.
Most brake boosters I've worked with won't even provide any boost below about 12 inches of vacuum at idle. The brakes feel like manual brakes at that point because that is what they become.