5/8" is stock bore diameter. Very close to 16mm.
16 15 or 14mm should work fine.
Smaller bore diameter = lever is easier to pull but it has to travel further to displace the same among of fluid as a larger bore diameter. Some guys say this gives you more "feel" to the brakes.
I have a 14mm dutch master. It works fine but I have to have the lever adjusted all the way out for it to work right. The clutch starts to engage/disengage when the lever is about 3/4" off the bars.
I haven't tried a 14mm master for the brakes. Some guys love them, some say you have to move the lever too far with it.
I don't know much about how radial masters would work with our calipers so I can't give you any input on those.
As far as a caliper swap goes, its hard to say without knowing what you're putting on. I've hard good things about a 14mm master and R1 calipers.