I made an experience with one of my old horns. I once planned to add a sheet brace to the bell bow and the tuning slide. So I first did the tuning slide. I'm not a trumpet maker, but I can do very clean solderings. So the thong was well done...took me a looooong time to fit the brace correctly to the slide.
I played the horn. 5-6 notes in med-low register. Hhmm, Tone looks to be a bit more centered, a bit better slotting. OK, let's try some upper notes , mid to high C. God.....what a disaster! The horn behaved exactly like you said: uneven tone, hard to keep in tune, hard tp play over the staff ( this horn is normally SO easy to play in high register ), and last but not least, sound just lost all its colors and harmonics.
Is it necessary to add I remover the thing immediately?
Adding a sheet bracing to a horn that wasn't designed t.o get some is a risky business. You cannot predict what will happen. OK braces make a horn looks cool. I was constantly looking for one before. Now I'm plenty happy with my good old looking trumpets!!