Will try to describe in this part what can be done to improve braking ability of our Supras. I want to make it clear from the very beginning that biggest and baddest brakes in the world will not stop any car faster if care is not taken about some other aspects, most important is tires. Sticky wide front tires will make a world of difference here. Bigger brakes are only for better heat dissipation (and for race car looks). If your brakes never fade with your driving style and you are happy with "feel" and pedal modulation than you don't need better brakes. Here are few hints on how to improve your car in this area
Mk3 Supra stock brakes are good for one emergency stop from 100 mph at best. Their size is absolutely not adequate for the weight of the car, they can fade after first or second high speed braking. First step in right direction here is to buy good brake pads. My choice for stock calipers was Portefield RS4 compound, they are great for street, do not require much heating, bite like crazy and dust moderately. I heard only good things about these pads.
Step number two is to buy SS braided brake lines, they don't expand under pressure as much as your aged rubber lines and make brake pedal a bit firmer. I use Goodridge lines, there are few other makes can be found, they all cost around $100.
If you are still not satisfied with brakes it's BBK time (big brake kit). There are few of them available, all of them front kits - KVR (Brembo components), MVP (utilizing Wilwood calipers) first come to my mind.
I have custom brakes made from components available here. For front brakes I used 4 piston Brembo calipers and brake rotors from BMW 7th series. The same calipers used on stock R33 GTR. Rotors fabricated with aluminium hats and weigh only 1 kg more than stock (7.5 kg vs. stock 6.5 kg). Rotors size is 315 mm, thickness 28 mm. A lot more braking power, and Brembo calipers are light years ahead of stock sliding 1 piston calipers. By the way stock cast iron calipers weigh 0.9 kg more than Brembo, so my kit is only 0.1 kg heavier than stock brakes.
Now to achieve good brake balance I had to improve rear brakes. For rear brakes I wanted to use either R33 GTR or mk4 Supra calipers, but due to long lead time involved decided to buy readily available used R32 Skyline GTR 2 piston calipers. As far as I know they are the same as 300XZ turbo calipers, at least brake pads are same. Nissan brake pads are smaller area than stock mk3 pads, but clamping force is higher due bigger piston's diameter. Rotors effective radius is also bigger than same of stock rotors, so braking torque of my new rear kit should be higher. I would like to emphasize here that brake pads area does not affect braking torque, advantage of bigger pads is just better heat dissipation and longer pads life.

For rear rotors I used front mk3 rotors with fabricated aluminium hats. At the same time when I was planning my rear brakes I decided to get rid of stock parking brakes. In the beginning I wanted to use stock parking brakes with aluminium hat, but after investigating this question (thanks, IJ) I learned that this setup would only work as parking brake only and if I ever decided to use it as e-brake or to block rear wheels at speed I would destroy pads and hats. One of my friends asked me why not to consider hydraulic brakes. This was excellent idea, this setup helps to get rid from a lot of rotating and unsprung mass in the rear. So, I left my car in fabricator's shop for rear and hydro brakes job. He also removed my ABS, stock ABS computer tuned to work with stock brake calipers and will be nearly useless and annoying with new higher volume system.
10 days later I got my car back with new brakes, hydro brake is great, it works with stock lever and block rear wheels easy. If this job was done again I would like to use smaller size master cylinder because brake lever stroke is too short, not like with stock cables/pads combo.
Overall brakes feel is not perfect yet, pedal feels a little mushy and difficult to modulate when fronts ready to lock. I am going to try first few simple steps to improve it. First to change to Castrol SRF brake fluid which is less compressible, than maybe I need front pads with better "bite", they are easier to modulate. Now I use Mintex front pads and Hawk HPS rear pads. If this won't help I probably have to think about changing brake pedal ratio or increasing size of master cylinder