Are kelly screamer mouthpieces good for trumpet?

i play jazz band, concert band, and marching band

I play a Kelly 7c and also own a screamer. Unless you are looking to pigeonhole yourself as a lead player I wouldn't recommend it. I played it for about a week and found that it made my tone very thin and although it helped with pushing out the ridiculously high notes, it made navigating the other registers a lot harder. It was also very hard to make the transition back to a regular mouthpiece which indicated to me that the screamer was developing a whole different group of muscles in my chops, so unless you are dead set on being a lead player, I would say go with something else.

I know a kelly who is a screamer.

Man she had a good mouth piece.

Never heard of them I play trumpet and flugelhorn and trombone and use Bach mouthpieces for all. I use the Bach 5A for all except the trombone which I use a 12 B.

Could be, but how would you ever know until you order one? My advice, find the nearest reputable mouthpiece maker and go to the factory with your horn. Test and test and test and test until you find what you like the best. I did this at Warburton about five years ago and haven't changed a thing since.