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I ride on disc equipped bikes, modern v-brakes and deep-drop calipers but I find these Mafac to brake strongly... If you have machined rims. The thing is: people often associate poor rim adherence and heat dissipation with the brake itself. It does brake pretty well with salmon pads. Not to mention that I'll of course swap the brake housing for something stiff and good quality cable. They are a PITA to set up correctly though, like many cantilevers, and are prone to brake sounds.
I ran one of these for London commuting back in the mists of time. The roller wears smooth and eventually fails to grip the tyre tread reliably, especially in the wet. It's also heavy. Hub dynamo is the way to go. Koolstop Salmons do improve the braking performance of Mafacs, but from a very low base. You are a brave man to ride them on long Audaxes! Modern deep-drop calipers are much more effective.