A smaller M/C will give you less travel on the pedal for a similar effort.

I'd guess Ford increaced it to 25mm because of the front pad deflection from the bigger piston on the S2. By having a bigger M/C (allbeit only 2mm) the fluid displaced in the pad deflection on cornering will be taken up in the system slightly better.

It may also work better on the S1 because of the smaller rear drum, the hydraulic ratio is balanced as originally intended.

If you drop down a S2 onto a 23mm M/C you may get slightly worse pad deflection thats all but I'd guess that 95% of road car drivers wouldnt spot it.

Hope that bullshit helps