There's a similar relation to speed vs the extra weight some of our 80's have. Now I won't have an exact formula to convert x amount of speed = y amount of extra weight. But both in the end are extra load on the braking system. Also keep in mind these brakes are designed for environments 30 years ago. We're driving faster, with more cars on the road now more than ever, and the need to modernize the brakes is highly prominent. Look at all the brakes on modern 4500lb+ SUV's, Trucks, or even large sedans/wagons. There are crossover's that have better braking system than our 3 decade old 80's.
I've gone through calculating optimal brake bias for my vehicle, and you can't have a balanced brake system by increasing front braking power and not address the rear accordingly. I have very lengthy and more detailed info on my big brake thread if you feel like having a gander.