Stan and Al are on the money here.
It's the repetitive heat soak that gets you in trouble. And brake balance is IMPORTANT too. The Airkewld kits can be had with 1,2, or 4 piston calipers in front, however the SAME Varga rear e-brake caliper comes with each kit. And we need an e-brake caliper on the street, don't we? I don't think there's a whole lot(none?) of engineering/calculation that was done there to be honest.
Back in the 60s, VW used a single circuit master with a 17mm bore. Then they changed to a dual-circuit 19mm. Later they used a 20.8mm dual circuit, which is the recommended size for 4 wheel discs.
But to what Stan was getting at, the sizing of the master cylinder(s), the brake pedal lever ratio, the sizing of the caliper pistons(and relation to pad size), and difference between F-R to get balance are all important. I never even questioned this when I bought my brakes, just bought the best braking available at the time. I lucked out, as mine worked out with a very good balance, no tendency to lock front or rear permaturely, and no tendency for the back of the car to come around either.
Drum brakes can work reliably, even in a racing situation, when sized appropriately. Formula Vee cars weigh less than 1100 pounds WITH DRIVER and work just fine. They are usually equipped with racing shoes and two separate master cylinders with a mechanical balance bar for bias. That's a far cry from a typical Speedster on the street, at typically TWICE that weight.
I wanted 4 wheel discs for two reasons: better and fade-free stopping, and not having to adjust brake shoes. I added braided flex lines for a harder and higher pedal. I also added more aggressive Wilwood front pads for higher temp resistance and better "bite". I think I've got a good package.
All braking systems should be able to lock all 4 wheels mechanically. It's the ease of doing so, coupled with repeatability and low maintenance. All discs cool better than drums(except for Porsche 356 aluminum-finned). Functioning front brake ducts(standard on V Spyders) are a nice feature. Easily do-able on almost any Speedster.
At the end of the day, the tire's contact patch and tread stick are the limiting factors of braking, besides driver skill.