About drilling out mouthpiece throats
Making a mouthpiece throat larger will make it longer as well as it cuts into the backbore.
Both mpc that you mentioned have the c backbore and could go up to a 22 throat. I play this but I create so much resistance that I can’t play any Bach horns. I have to play Calicchio, Callet ect.
You might try going to a 25 or 24 throat. Or you could try relaxing your corners and pivoting the horn away from your top lip when playing over high c. There are several things that add to the overall amount of resistance. Mouthpiece pressure, lip compression, lip curl, tongue arch, corner tension. Sometimes backing off on one of these will really free up the upper register.
Also Giardinelli mpc project more than Schilke because they have shaved shoulders between the cup and throat. I have this done on all my Schilke mpc.
I would be cautious with a 19 throat. The bigger the hole the longer the throat is. This long throat takes the front part of the backbore out. This causes many problems with intonation. Even if the backbore were recut after boring out the throat it could be fixed only part of the way (unless it was filled in before it was cut). Think about it if the throat is 1/8″ longer then the backbore taper needs to be moved 1/8″.
Dynamics are also a problem. Some players actually need to play softly from time to time. Huge throats make this soft playing all but impossible. That big a hole is going to be hard to keep a spin on the notes.
Usually 27 throats are bored to 26 or 25 without a backbore change. I personally consider 22 as big as I would ever go on a throat and that needs to be cut that way not bored out. I believe in open equipment but you are heading toward the baritone range. 😉