At one point I had searched the web for the GR mouthpieces and couldn't find any reviews on them. Doesn't mean they're not out there, I just couldn't find them.
The Morgan mouthpieces dont necessarily target the Desmond sound. However, they are made in a similar fashion to the vintage MC Gregory pieces: medium round chamber, scooped sidewalls, rollover baffle.
I would recommend the ML chamber in a small tip, and possibly request a slightly lower baffle profile. But, the stock piece as-is will tackle the Desmond stuff with no problem with a stiff reed. That being said, so will a ton of other pieces. It's much more about approach and style.