Hi Adam.
FWIW OEM, The ZTL2 uses a 19mm M/C and IIRC ZTL caliper uses a 18mm M/C.
I can tell you a ZTL M/C with a ZTL2 caliper is unusable. It's so soft, there's not enough piston stroke to brake lock, so I can confidently say don't vary the M/C size by more than 1 mm. Could you even find a 20mm M/C?
IMO, if the system is in good working order, you are much better off playing with different pad compounds. Motorcycles are much more sensitive to these ratios than cars are.