I am with David, here. AT FIRST, I didn't notice a range increase, just endurance. Now that I have optimized their use, my range went up a bit, too.
However, I am at my wits end when it comes to trying to help folks play them. It did take an adjustment for me, and I went through a period (as recently ast two years ago) when I had trouble switching back and forth between my 342 and a conventional mouthpiece. My lower lip seemed out of whack.
Now, I have developed a full blown methodolgy around the use of the lower lip, and I am very comfortable switching my mouthpieces and horns.
Not everyone can play the 342's. It has been my experience that folks who play with a downstream set (the horn pivots downward as you ascend in pitch) can adjust nicely to the 342, but folks who play dead level in all registers or those folks who play with an upstream set have moe diffictuly making these work.
I do have a couple of video clips with my efforts to help folks play the 342.
YouTube - How To Use an Asymmeric Mouthpiece YouTube - Asymmetric Part 2
I hope this helps a little.
Peace!
Nick