After having owned a 450 for 10 years and 150,000 km's, the rear drum brakes
still had 40% remaining which, tells me they weren't used very much...
Exactly, and it can be felt when braking hard and transitioning into a corner. A brake proportioning valve can be of significant help to correct the braking bias.
But the other issue is that drum brakes don't require as much brake fluid movement as drum brakes, so swapping to rear discs without changing the master cylinder can result in little or no braking power. Worse is that some drum systems used a valve to retain some residual pressure in the fluid, and that can cause a disc brake to drag. Remove the valve and braking becomes very weak as the master cylinder can't move enough fluid to pressurized the line and clamp the discs.
It's worse with the ED that I have. Although the braking imbalance is partly made up for by regen, when the yaw sensors experience forces greater than expected in normal driving, it thinks a crash is imminent and then cuts the regen entirely.
With a regular fortwo, a proportioning valve to decrease front bias would seem likely to work well as long as the owner is willing to accept a bit earlier ABS intervention.