Mouthpieces do not play horns -- Players play horns.
Mouthpiece discussions = Nasty can of worms !!!
Bottom line - A mouthpiece will not "give" you range. It will "help". If you are not a high "G" Double "C" player on your current MP, a MP change will not magically get you there! Arturo Sandoval plays on a 3C, Maynard played on something closer to a French Horn MP, Doc played on a 13A4A and Bill Chase played a 6A4A. Now that is a WIDE range of mouthpieces !!! All lead players. Some professional horn players change mouthpieces depending on what type of music they are playing, some do not.
Comfort is what you need to look for in a Mouthpiece, comfort that "helps" you achieve your desired sound. Comfortable may be big and shallow or small and deep ! It could even be the mouthpiece that came with the horn, or the most expensive MP Monette makes !
Do not get too hung up of finding the "perfect Mouthpiece" because it does not exist. All mouthpieces are a design compromise, Find a mouthpiece that is comfortable, helps you achieve the desired sound... Then PLAY IT !!!