@fBoyle-- I'm exactly the opposite. I play jazz so I need to go high range with a loud projection. So I use a small mouthpiece, a 12c. It works great, especially when I'm playing solo.
I actually would like to know what type of a trombone mouthpiece artists use when they frequently switch between instruments. I think some trombone players also play the trumpet. I wonder if this affects the mouthpiece they use for the trombone.