Has anyone ran a pom master cylinder booster combo with needed pedal box in a fast mini
Is it a worthwhile thing with any gain please

Not quite sure what you mean by gain but they're a bit more bulky than the original m/c and booster set up.

I will translate for him as he is a top tv tech but not mini savy as he was in the 80's he was looking for info on the Rover mini brake setup with cylinder booster in one is it as good or better or worse than a clubman gt style for stopping and wondered if Rover mini gained more room on inner guard

What happens when you need to service or repair that damn thing miles from anywhere?

Stay with a VH44

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"Show me the Mini!"

Personally I cant see why it would stop any better the a S or GT setup. Any booster will reduce pedal effort. It will still be in the way whenever you want to work in there! To me its just more linkages and modifications required for the same job.
I would look at pad compounds and calipers if looking for improvement.
My thoughts

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1963 Morris 850 Smoke Grey
1963 Morris 850 Shadow Blue
1965 Morris Mini Smooth Van
1966 Morris Mini Van
1969 Morris Mini Deluxe Resto
5/72 Clubman Gt