The main reason for the low break force lays in the difference between the two pistons (lever versus caliper). Honda was experimenting in that era, this was one of the first disc brake setup, and is actually the same as on all 450's from 1970 and up. The disc itself is very thick and heavy, the mass alone will make it difficult to heat it up quickly. Eventually, it will heat up, but that takes a serious amount of breaking, way more then everyday usage (at least that is my experience wit this bike). The disk itself has a smaller diameter then modern set-ups, resulting in a lower torque-arm with respect to the centre.
Perforated disks don't brake better, probably worse, because there is less surface to be used. For breaking in a wet environment the perforation will help to get rid of water and dirt. Most brakes from cars are fat disks, not perforated and enclosed by the mudguard and wheel, more or less the same as the Honda CB400f. Most cars have multi piston calipers, which making the friction surface larger, resulting in more stopping power.
btw, I made some pictures of the wires from the generator after cleaning:
Overview of the generator wires after cleaning and repair
Close-up of the generator wires. Not happy jet, can be cleaner. Now soaking in silicon fluid, tomorrow they will be cleaner.
One out of 4 boxes of Honda wires, taken from harnesses like explained earlier. Also a few modern wires in the Honda colours, mainly High temp wiring for generator repairs.
Orange winker wires with bullet contacts (male and female)
Light blue winker wires, also with a choice of connectors
Complete sets for certain older Honda's
CB450 K1-K7 wire harness parts
Very rare woven umbilical for the plus wire for older Honda's (CB450 K0, CB72 etc..)