Ive been playing on my schilke 51d for 3 years now and im still having issues with high notes and response... in comparison to the 51d what mouthpiece am I describing:
Wide rim, large shallow cup, and a small throat
When you say wide rim, are you referring to the inner diameter or the actual width of the rim?
You might try looking into some trombone mouthpieces. Greg Black and also the Alessi signature line both offer wider rims with Slater cups and backbores