You generally find that the bore sizes between master cylinders differs between those for single disc and twin disc.
If I use the master cylinder off the 660Z with a single disc setup the resulting amount of pressure you have to use at the lever becomes excessive and the resulting loss of feel at the lever usually described as feeling wooden.
Having built and repaired many race bikes in my lifetime I have experienced this problem many times.
The bore in master cylinders differs in size from bike to bike, usually dependant on single or twin disc.

Pete.