I am an amateur player with a British based Windband. I juggle between playing Trumpet, Flugel and Cornet consequently it is more than useful to use a mouthpiece of similar dimensions for all the instruments. The trumpet I play is a Bach Stradivarius (excellent), and I have been more than pleased with the Bach 7C they supplied with it, I consequently bought Bach 7Cs for both my cornet (a Virtuosi - not recommended :( ), and my flugel (a Cerveny 501 - good for playing when I visit my German friends). The cornet was fine mouthpiece-wise, but I was not aware of the difference in shank dimensions on flugels and was disappointed to find that the Bach 7CFL did not fit the Cerveny, which is where the JK comes in.
I have found that the JK plays well and true from below the stave to way up above the stave. When using the Cerveny supplied mouthpiece, when playing in the lower register, note entrance was either fluffy or over-hard through trying to compensate, the JK mouthpiece allows me to D-tongue easily down to a low A.
Prior to receiving the JK mouthpiece, I had used a trimmed down Kelly 7C trumpet mouthpiece which although helping in the higher registers, also hardened the tone. The JK allows me to fly up to the top B while retaining the Flugel tone. I am still practising and am sure that I will be able to push beyond high C carrying the tone with me. I appreciate not many Flugel parts are written this high, but in practice with a band at Weeze, Germany they introduced me to a piece written to top 'D' on the Flugel part - yes I struggled to squeak it out :)