Schilke Trumpet 15A4

This is bigger mouthpiece with shallower cup but still open (c bore) feel.

Rim num. 4 is very comfortable and little bit more rounder than on 15C4 piece, which sustain flexibility and endurance on this mouthpiece.

If you are looking for endurance, comfort and help with upper register, this is right piece, but sound is more edgy and brilliant because of A cup.

I really like c bore, because of open feel and that's why this mouthpiece can be used for flexible playing in wide range of register and music styles even as shallower piece.

You really can not compare this mouthpiece to 14A4c piece (which I also like with c bore).
15A4c piece is more comparable to 15B than 14A4c. 15B has more rounder Rim and much more open feel, but less resistivity. Standard 15 is nice piece but really different in shape and feel than mentioned mouthpieces, but beautiful production sound.

Remember, mouthpiece can help, but can't train you.