As with all trumpet mouthpieces, they are very personal items, as such I will try to make some objective comments as well as subjective.
Firstly, I make a living playing and teaching. I play mostly commercial music and I have a very dark sound naturally. I have always gravitated to shallower pieces to help produce a trumpet sound rather than a flugel-y sounding trumpet. I play on a 1600i XO Roger Ingram M bore trumpet.
The notion of small and shallow is strange to me as I have large lips and find it easy to play on this mouthpiece - granted this was not always the case. I would probably compare this to a Bach F cup, so it is shallow, but for me I would love 10 or 15 thou shallower. It is very difficult to comment on the depth as the shape of the cup is crucial to the playability. The resistance of this mouthpiece probably makes it one of the easier shallow mouthpieces to play.
For me, this mouthpiece doesn't sound shrill or thin. I like how it plays and I like how I sound on it, albeit sound is always possible to improve upon.
For those who are coming from the Marcinkiewicz Shew background, I can shed some light. I too played the Marcinkiewicz Shew pieces, but I always found they played quite sharp on the pitch. My assumption was that the backbores were too tight or the cylindrical portion of the throat was perhaps unbalanced. The Yamaha seemed to fix this for me: tighter backbore, and now the tuning is much improved. The Yamaha projects very well and seems to be somewhere between the 1.25 and the 1 Marcinkiewicz Shew. The rim profile is different, but still very comfortable and rather rounded - again I like this. The bite is very soft and the alpha angle high.
For the money I don't know why you wouldn't try one!! It has worked out well for me. Part of the knack to making these smaller/tighter pieces work is to really back off and don't blow so hard, you don't need to!
Anyway, that's my 2 cents, for what it is worth.