I switched from the Schilke 51D that I used on the Courtois 165 to the SM3-SM3U-SM3X in the order on the Besson 2052.The SM3U did not really like me; the SM3X is a good mouthpiece with a brighter sound than the SM3U. Then, a couple of years ago, I wanted to try K&G mouthpieces and, with these, I found the most performing mouthpieces I've ever tried: excellent projection, sound, intonation and resistance, so much so that I also use them on the baritone and trombone. If you can I suggest you try them. The mouthpieces are like shoes: you have to try them before buying them.
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.