when I studied with the Hamburg Conservatory back in the 70ies, I first was given a refurbished Bach 42B, which my Prof. had in use while performing with NDR Symphony. The teachers recommended Giardinelli 4D mouthpieces for the students. After a while, they wanted me to take an 88H instead of the Bach, background was sound and better playability (?). Anyways, I still was not on the Conn Remington, kept going with the 4D. Later on, I decided not being good enough for going professional in music but continued playing trombone on a semiprofessional basis. Years later, I got hand on a 1964 Conn 88H from Staatsoper Hamburg and had it for a long period of time until life had beaten it up. Still played the 4D. I never was too happy with that combo but could not find out for anything better.
Fast forward: A couple of years ago, I decided to replace my small bore trombone with Conn 88H or 42B, again. Got the LT42B. Had Giardinelli 3G on the 42, much better than the stock 6.5AL, which I put into the drawer. So I did with the 42, it turned out to be not my cup of tea. And in between, I mainly was on trumpet.
Since I retired now from my daily job and the trumpet, I pulled out the 42, still in as new condition, it only had played 1 concert with a wind players symphonic orchestra.
And - nothing worked out as it was supposed to. The large 3G now was too much for me. A 12C Bach with large shank I have is a bad idea for the large horn. And the 4D with that wacky adapter? No not really, a 3D with an "F" - rim is very nice, has an 104 diameter / 26mm. But in return, it comes with a tubby sound.
Allright. Not satisfied with any of my trombones with the exception of the brand-new 354E Yammie, which runs on Giar 6D.
Now, I'm stubborn. IF the R&D at the factories is not working mindlessly, there must be a good reason for them to provide the new horns with the mouthpieces they put into the cases.
These two mpcs were not particularly comfortable in the first run nor did I like the sound, but after a while of settling in both of them turned out to be a very nice fit. Absolutely. Original 6.5AL on Bach 42, original King 7C on 3B/F. The 7C is not the best choice for me due to the sharpness of the inner edge of the rim, but it plays very well with the 3B and took away the quirks from the range around high Bb and up. So did the 6.5AL on the 42. Remains of a disturbing vibration at high Bb disappeared completely (it was mainly caused by poor greasing of loose parts of the valve and of the tuning slide). Both horns play nicely in tune and offer their typical sound.
My results: Try the original stock mouthpiece if nothing else helps for satisfaction. Maybe, you can take advantage of doing so.
My best guess is, the R&D guys at the major brands knew what they were doing, possibly the delivered stock mouthpiece is part of the instrument design in mass production surroundings.
Sorry for the length of this, but I've got some time to write sth boring for you...

Greetz, John