RC51 brakes are pretty sweet as is! 4 pot Nissin's do the job. Some argue that the radial 4 piston Tokico are not a better caliper, but I don't know for sure.
I'm no engineer, but it seems to me the calipers are mounted to adapters, which are not radially mounted to the forks. So you don't get any of the benefits of radial mounting calipers. The adapters still will allow flex under braking forces.
IMHO the only reason to mount adapters is aesthetics. If that's what you're after, more power to ya!
Radial Brembo master cylinder with the stock RC51 setup, however... There's your bigest gain.
Sorry, I have no experience with ABS. So nothing definitive to add there. Does it make a difference to the ABS system if there are no longer all the extra hoses, pistons, and PCV associated with the stock linked system?