For the past year, I have been using a Yamaha 48 mouthpiece with my small bore trombone. Despite being very happy with its performance across the upper registers, I became increasingly aware that my low end was sounding shallow/pinched.
I recently purchased the 48D to try and remedy this.
As expected, low range sounds much fuller and is far easier to pitch. Ease of playing in the upper register has unfortunately decreased as a result of the deeper cup, but I am hoping that with time (and practice) this will become less of an issue.