“. Several previous posts from people who bought and fitted them say they don't give any more power at low rpm than standard RM21.”

I was wondering if you have anything more rigorous than anecdotal evidence for this comment?

The power curve at low rpm between an RM21 and original RM23 will only differ very slightly. It would require accurate measuring instruments and good experimental method to graph this small difference, and also to show any difference with the “genuine” Lucas RM23 at this low rpm part of the curve.

I very much doubt that anyone without such equipment/technique could convincingly discern the difference between any of the above mentioned stators at low rpm.
The differences will of course be much more obvious at high rpm with ordinary ammeter/voltmeter.

If low rpm performance is of importance, which I think it is for ordinary road-bike bigger twin users, the best (Lucas) answer I agree is the high output RM24.