I found a screen shot of the CBR600RR factory service manual from the correct year that my Nissin brakes came from. It showed 32mm/30mm caliper pistons under the specs/service limits.The 4 pot calipers I researched were 34/32 and 34/30 respectively. Better make sure what you really have.
Edit: Found another source that suggests 31.8mm & 33.68mm. Maybe you're right.
So it looks like an older 5/8" V-Strom master cylinder would put it at the perfect MC/caliper ratio (right at 17). I'm at 21.9 right now with the 14mm MC. The radial one is tempting too.