Well, I'll jump in here. I don't think any diesel or Turbo diesel A1 or A2 needs anything larger than the 9.4" fronts, and the stock rear drums. In my experience, the stock VW brakes in various sizes and configurations are excellent for the cars they were delivered on, and good for a bit more performance. Here's the way it generally breaks down:
9.4" solid front rotors with 180mmx30mm rear drums: These are the lightest weight, and
with proper pad selection they can handle any car up to 2400 pounds or so gross weight at speeds up to about 100mph. They do tend to fade a bit toward the end of a 70mph stop, especially with cheap pads. But they are excellent when used with good performance pads selected to match the driving conditions you will encounter.
9.4" vented front rotors with 180mmx30mm rear drums: These were on the Rabbit GTI's and 8V Sciroccos. The vented front rotors were also used on a lot of the A2 GTI's. These can handle higher speeds, and if you use them hard at higher speeds, you need high performance shoes for the rear drums and better front pads. These are excellent for up to 2400 pounds gross weight at speeds up to about 125mph.
9.4" vented front rotors with rear discs: This was the setup for the early A2 GTI's. They are good for about the same loading and speeds as the 9.4" vented fronts/180mmx30mm rear drums. They are excellent for up to 2400 pounds gross weight at speeds up to about 1250mph. In the mid 1980's, when friction material technology was limited, the rear discs were good for fighting rear brake fade in extreme use. Today, with Porterfield R4-S (street) and R4 (race) compounds available on the 180mm shoes, that's less of an issue (provided you use good shoes in the rear drums).
Proportioning for the rear discs is a big challenge for the A1 chassis. The A2 proportioning setup is good for A2 cars and makes rear discs much easier on an A2. Also, the VW calipers have very unreliable parking brake mechanisms and they weigh about 6 1/2 pounds more than the drum setup. The added weight is unsprung (actually unsprung, beyond the wheel centerline, so it's about 8 pounds of load on the springs and shocks). You do get slightly less rotating inertia with the rear discs, so acceleration is slightly better.
10.1" vented front rotors with rear discs: This was the stock setup on the 16V Sciroccos, and on the 2.0L 16V Jetta GLI's and B3 Passat 16V's. You have to use the 22mm master cylinder with these, and proportioning is an issue for the A1 chassis. Also, 14" is the smallest wheel that will clear the front calipers. These brakes will handle up to 2500-2600 pounds gross weight at speeds up to 135mph-140mph.
10.1" vented front rotors with 200mmx40mm rear drums: This wasn't stock on anything, but for an A1, these match up well. You can use the drum brake proportioning valves with this setup, and the balance is pretty good. Good for up to 2500 pounds gross weight at speeds up to 135mph or so. High speeds require high performance shoes for the rear discs to avoid fade issues at the rear.
I won't go any larger than that because it's not very applicable to diesel powered cars.
The big weight penatly with the larger brakes is in the wheels and tires. With light 13" wheels and light tires, it's possible to get the wheel and tire package down to 25 pounds or so. With 14" wheels and tires, getting under 35 pounds per corner is difficult, and the lightest I've seen is about 30 pounds per corner. 2 pounds on each wheel is about the same for acceleration as a pound on the flywheel. The added inertia from the larger wheels and tires is very significant.
On my 1984 diesel I run the vented 9.4" front rotors, and I actually have rear discs that I've been screwing around with for about 6 years. I have another Jetta here and I might just go back to the rear drums on the diesel and put the discs on the other Jetta for when I sell it.
Although, with extreme mods (rotor weight reduced by about 40%, aluminum calipers from an A4 chassis car, modifications to my custom brackets for the A2 load sensing regulator, etc.), I have the rear discs to the point where they are about the same as the original stock drums in terms of handling, and acceleration is slightly better. But unless you want to do a whole lot of work for very little gains, I wouldn't recommend rear discs.
At the front, I don't see any IDI diesel needing anything more than the stock brakes. I run the vented front rotors at minimum rotor thickness (or slightly below minimum thickness) because I have worn rotors from the race car, so it's more for convenience than for any real improvement in braking. At minimum thickness, the vented rotor is about the same weight as a full thickness, new solid front rotor. The vented 9.4" rotors are a decent upgrade for a car with the 9.4" solid rotors. You get about twice the surface area for cooling air with the vented rotors.
I see at least one Quantum owner posting here, so I'll make a comment or two about that. If you have a Quantum and need larger brakes due to additional weight, the 10.1" fronts with either rear discs or 200mmx40mm rear drums might be a good choice. The Scirocco 16V/Audi 4000CS caliper carriers will bolt right up to the front of a Quantum (or Dasher or Fox, or Rabbit or A1 Jetta). But you'll need 14" wheels with the 10.1" rotors, and that adds a lot of inertia to the package, hurting acceleration.
EDIT:
Also, if you want to use an adjustable proportioning device, for a street car you need to run two separate adjustable valves, one in each rear line, or get some kind of dual channel setup. Dr. Diesel's dual prop valve setup is a good example.
This setup
works for a track only race car. It looks like he has a tee connector joining the two rear brake lines into a single adjustable prop valve, and then another tee connector splitting the combined prop valve back out to the two rear brakes.
That setup is not street legal! Every vehicle made since the mid 1960's (1964 I think) is required by law to have two separate and independent hydraulic circuits. That way a failure or leak in one system will only disable the brakes on two wheels at the most. With the setup in the picture, a leak in one caliper will disable the brakes on all 4 wheels. Again, if the rules allow it, that works fine for track use, but it's not a good idea for street use.