I did an article years ago for one of the Pontiac magazines. I tested a manual drum brake 68 GTO against a power disc brake 69 GTO in 60mph-0 brake testing (using a Vericom performance computer to document results). Here's what I found:
First baseline tests were with both cars in the "as-received" condition with well-used, but servicable, brakes. The drum car initially stopped within just a few feet of the disc car, but stopping distance starting increasing significantly after the third 60-0 test (tests were done quickly back-to-back to get the brakes rediculously hot). The disc brake car outperformed the drum car by a bit of a margin after the brakes were really hot, but not when the brakes were cool.
We then did a complete brake rebuild on both cars. We turned drums and rotors, installed pads on the disc car, and installed all new springs, shoes and hardware on the drum axles. We used top-of-the-line semi-metallic shoes (they were Accel shoes - since discontinued), and we bled the brakes.
With the good quality shoes and a fresh rebuild, the drum car performed identically to the disc car, even after 6 (six) 60-0 fast tests. Braking distance was limited only by the driver's ability to modulate the brakes to avoid complete lock-up. The drums never faded with the new hardware installed, and both cars stopped straight, safely, and with no pull or erratic behavior. We actually did several of the drum tests with hands-off the wheel during braking.
You'd have a hard time justifying switching from drums to discs from a safety point of view if you have a well-rebuilt set of drum brakes with good-quality shoes, unless you're involved in some type of competition driving where you're going to get the brakes glowing red hot for a period of time. On the street, discs look cool, but that's about the extent of the advantage.:cheers