I would be asking him why. The Yamaha 16c4 according to Yamaha's mouthpiece chart is equal to a Bach 1 1/2C, and yes that usually is considered a pro mouthpiece. If you have a dark sound on it and it gives you the range to play what you need to play then I would not see anything wrong. I would press him for a more definitive reason as to why. What type of style of trumpet playing does he do and does that type of playing fit with what you want to do? If not, find someone else.