This backbore is a superb match to the M, S, ES, SV and ESV Warburton cups as it tapers to a 28 bore before opening back out. In my opinion it is this added resistance that helps minimize lip intrusion on the more specialised shallow equipment and allow you to best play on this gear.
The sound is bright, focussed, projecting and well in tune. While I used this gear I paired it with a 4S and was very pleased with the sound it produced. It is easily my favourite backbore along with the KT*.
Warburton equipment has always been well made and finished to a high standard and this was no exception. For a commercial player it is a definite weapon.
I used it once with a 4XD top to play the Messiah on a rotary piccolo as I wanted some brilliance to the sound without overpowering a small ensemble. The resistance helped me back off and create a (hopefully) pleasing tone.
Worth a shot!!