I'm a small guy (1.64) with small lips and teeth, and have been downwsizing from a 13A4A to improve both range and endurance. First I tried a 6A4A for several months, however the 16mm rim diameter was still too wide and the combination of shallow cup and tight backbore was putting up too much resistance right in front of my chops.
A friend lent me his old Warburton 9ES and a #5, 6 & 9 Backbore, which offered some material to experiment. The Top was just a shade too large and the 5 & 6 too closed. The #9 was open enough, but intonation was comprimised. Took an educated guess and ordered a 10ESV and mated it to a #8.
This is for me the best all arounder I can find, and the projection is immense without having to make comprimises in articulation, intonation or sound. The shallow cup does not back up as much because of the V-Cup, and I feel that the resistance has moved further down into the horn which makes it much easier to play higher for longer. For me.
Facit: The Warburton system gives you the chance to experiment with almost an infinite number of combinations, until you get exactly what you need to get the job done.