I am unfamiliar with RHD master cylinder applications. However, in a general sense, the bore diameter differences you describe would not significantly impede interchangeability. In theory, all things being equal, smaller bore diameter would affect pedal effort so that increased pedal travel would be necessary to obtain the same hydraulic pressures (less firm or softer pedal). Pedal effort preference, not unlike steering effort (steering wheel size), seating position and even shifting effort preferences can vary between drivers.
If I remember correctly, the 2800 master cylinder intended for use with rear drum brakes had a small ("anti-reversionary") check valve bolted to the unit, in between the rear brake tubing. (Some sources refer to this valve as a "pre-pressure valve" pn: 34314650332 ) If I understand it, this valve was designed to maintain slight pressure on the rear brake lines to limit brake shoe contraction and keep them close to the drum surface. This, of course, would serve no purpose on the 4-disc model and I would imagine that the valve could simply be left off of any 4 wheel disc application. A remote secondary issue might involve the rarely-touched proportioning valve at the rear of the vehicle. Practically speaking, the unit ensures that the front brakes preform the majority of the braking. Assuming that your proportioning valve is correct and properly functioning for your 4-disc vehicle, one would expect no significant difference in overall braking action (except for previously mentioned pedal effort) if you used the smaller bore master cylinder.
So-called "Pre-pressure valve" commonly found on 2800 master cylinder
