I think you really have to try it (and you can get it on trial from Mouthpiece Express or -- my preference -- Hickey's). Overall, Yamaha mouthpieces seem to be a little smaller/shallower than the corresponding Schilke piece. As an example, I recently tried a Yamaha 60 (from Mouthpiece Express) hoping it was smaller and more controllable than my Schilke 60. It was -- noticeably different in response and ease of playing. I almost decided to keep it, but instead decided it wasn't quite what I wanted and sent it back. I do like the Yamaha mouthpieces. They're very well made and, as you point out, relatively inexpensive. I also almost kept a Yamaha 60BL which is a pretty strange beast (basically a shallow 60 with a much more comfortable -- wider -- rim). It gives a very bright sound and is in the genre of German trombone mouthpieces. Would be great for jazz/commercial bass trombone. I should have tried it on my euph, but didn't.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)