Kelly 7C vs Yamaha CR-11C4S
The Kelly 7C has a wider cup diameter, the Kelly 7C has a larger throat opening.
Cup Diameter
The Kelly 7C has a cup diameter of 16.51 mm / 0.6500 in compared to 16.46 mm / 0.6480 in on the Yamaha CR-11C4S — a difference of 0.05 mm / 0.0020 in. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
Both mouthpieces share a medium cup depth, so the sound character from depth alone will be very similar.
Throat Diameter
The Kelly 7C has a wider throat (3.66 mm / 0.1441 in vs 3.65 mm / 0.1437 in). A wider throat allows more air through, increasing volume and projection but reducing resistance.
Material
The Kelly 7C is described as Lexan polycarbonate, and the Yamaha CR-11C4S as unspecified. Material affects mass, warmth, and how quickly the piece responds.
Which is better?
Neither mouthpiece clearly dominates the other — the right choice depends on your embouchure, playing style, and the sound you're after. Try both if possible.