It's quite some time since I worked on my MC21 carbs with HRC settings.
If I recall correctly, standard road setting jet sizes were something like 128 mains, then with power jets coming in at high revs.

HRC SS/SP settings were increase main jets to something like 188, and blank the power jets. As the main jets are much larger than 'stock' the power jets are no longer needed.

I'm struggling to figure how slow jets could be casuing your running problems, as long as the slow jet is not blocked / clogged, this shouldn't present any other porblems.

I can say though, that you have 100% perfectly described my early starting / running problems with my MC21SP, and that was a knackered crank outer seal.

When you say it is definitely not an air leak, I take it you have done a full leakdown test on the crankcases, blanking off the exhaust and intake manifolds, and have tested pressure both ways?
_________________
Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30