I spoke with Keith earlier with my view on the disc conversion, and while it's just my opinion, it is backed up with countless posts and conversations here that seem to add up if we look at them correctly. It's a matter of pressure, volume and placement of better parts being wholly impractical without re-engineering the pieces forward of the pedals.
Hypovolemia prevents all four corners from grabbing at the same time. So what do you do? Skip the discs in the back. Proportioners don't work if there's a stock MC (talking about the CB kit, specifically) and pre-loading the back is tricky anyway.
It isn't a matter of being able to convert an IRS or swing-axle Beetle brake system to rear discs, either. We know that can be done with a JPS, a Vintage, a Beck or a newer Intermeccanica. What I'm very specifically referencing is the Classic Motor Carriages (and therefore early IM, Street Beasts/FiberFab cars with the 2"x4" perimeter box tubing and thick-assed metal arch that prevents the use of a decently-sized improvement on the stock Beetle master cylinder.
The arch that rises behind the pedals, over the tunnel and down the other side is a pretty serious piece of steel. Go ahead and cut that if you want, but you'll have to fab up something to replace it with. If you're doing it to replace the Beetle MC, you're going to interfere with either the tire or the travel of your pedals.
Complicating matters, if you've done anything to narrow the beam up front -- or are using wider than 5.5-inch wheels with the negative offset toward the center, there just isn't any wiggle room. You'll pretty much screw up the front of your car.
As haphazarldy as some of them are put together (the CMC jig must have been crooked, if they used one at all), that's still the cornerstone of the car's strongest structural element. I wouldn't like to mess with it without adding similarly strong materials in place of that arch-and-box complex, and I don't think the average VW enthusiast wants that kind of PITA.
I tried the disc route in order to stop my 172-horse car in a straight line, and it was squirrely no matter how I attempted to fix it. I spent my time, energy and money on parts, then went to Tif's in Annapolis ($70-is bucks an hour for a day's work), AutoFab Race Cars (again, performance shop rates) and At Speed (a THIRD professional race shop) -- and NOBODY could figure it out.
I tried proportioning valves, new lines, two sets of discs and every other damned thing I could think of, and to no avail. Without changing that arch, I am limited by geometry to how much fluid I can push. So be it; I've gone back to drums.
The car once again performs normally.