. Several previous posts from people who bought and fitted them say they don't give any more power at low rpm than standard RM21.
The power curve at low rpm between an RM21 and original RM23 will only differ very slightly.
'original Lucas' advertised 75% of rated @ 2,400 rpm for a single-phase like the RM23,
... 'original Lucas
' rated the RM21 for 10.5A @ 5,000 rpm and, as I say, the RM23 for 14.5A @ 5,000 rpm; 75% is 7.875 Amps and 10.875 Amps respectively, an extra 3A at low rpm is a bit more than "differ very slightly" on these old heaps.