This is fascinating. Love this discussion, the illustration is awesome and bulgie's explanation finally sealed my opinion. If anyone wants to entertain an example...
So I have a ~94 Record 8s RD that I swapped the short Record OR cage onto (cheap, gets a little more wrap, and only one of any length I could find). So if I keep my upper pulley the same, I can get the biggest rear cog size possible. If I increase the size of the lower pulley, I can potentially add a tooth or two of additional wrap, right?
I haven't got this on the bike yet as I need to do some frame de-rusting and then figure out which shifters I'm gonna use (8s Record through and through or 10s Chorus with a 10s Shimano cassette?). But I may just have to play around with this, as I like the lower gearing on my other bike so much. Gotta start hunting on eBay for some 11, 12, or more tooth pulleys, and maybe some spacers to get the width right, and see what I can fit in there. Eyeballing it, looked like lots of room for more teeth.