Fontyyy, I've heard before that the MC28 cylinder has more conservative exhaust porting than the MC21? Do you know to what degree?

Honda have superseeded all the MC21 cylinder part numbers to a MC28 part number, just one size - 'A'.
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Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30