Unfortunately, I don’t seem to have taken good notes at Rob Naczi’s talk this summer, but his current (2024) estimate for the northeastern US is that the New Manual will cover about 20% more species than were included in Gleason & Cronquist (1991). As best I recall, the largest category of increase was from new non-native taxa. I don’t think taxon splits (vs range extensions, or the discovery of nondescript taxa) made up a huge chunk of the pie–10%? 15%?
The 33 years includes the invention of PCR and the development of molecular phylogeny, so I would not expect the same pace to hold over the next 33 years and that non-native taxa would dominate additions.
Some time I do want to sit down and do a Gleason & Cronquist->Flora Novae-Angliae->Flora of Vermont comparison to get some stats for the northeast but I admit it isn’t a high priority.