My bike started life as a 1979 R65 with a pair of silver ATE twin pot calipers (one pot each side of the caliper, working in opposition to each other).
However they needed some serious work so I swapped them for a pair of used Brembo twin pot calipers from a post 81 R65. However I did not change my master cylinder to the later type, so they were a bit firm and lacked feel, but were powerful.
Recently I have gone from a dual disc setup to a single disc using one of the Brembo's and an appropriate post 81 R65 master cylinder. The plan being to upgrade the single disc to a full float like Rob Valdez. I find that I have gained feel and progression, reduced unsprung weight, but lost outright power. This is not an issue as I do not have a large fairing, generally do not carry luggage and do not carry a pillion.
The bottom line is, always change the master cylinder to match the brakes! Unless you are very lucky indeed.
cheers