I have been playing for some time , in the past few years with 51 D, which suited the Courtois 165 well. Then, with the Besson I had experiences with the various DW SM and Alliance models, but none of them fully satisfied me. I found the ideal mouthpiece (for me) in the K&G brand, which I use regularly on my instruments. In my opinion they are the best mouthpieces, from all points of view, I have ever tried. I would give the K&G 4D+ a try, which has the internal diameter of the cup at 26 mm internal diameter of the cup, comparable to the Schilke 52.
2007 Besson Prestige 2052, 3D+ K&G mouthpiece; JP373 baritone, 4B modified K&G mouthpiece; Bach 42GO trombone, T4C K&G mouthpiece; 1973 Besson New Standard 3 compensated valves, 3D+ K&G modified mouthpiece; Wessex French C tuba, 3D+ K&G modified mouthpiece.