BW,
A good mouthpiece is a precision object where all of the factors - Cup, Rim, Throat, Backbore are related to one another. If one of the variables is changed, you need to change the others to keep the mouthpiece "balanced". Opening the throat does not make a mouthpiece necessarily more free blowing. It will change the intonation and slotting for sure, and not always for the better. Your range and endurance will not get better just by opening the throat unless you have a badly designed mouthpiece!
I am sure that there will be several posts to the nature: "I did it and it was like pulling the plug out". I have had several students with that attitude and we did a blind test a couple of years ago (real blindfolds!) and EVERY SINGLE ONE preferred the originals to the bored out ones. Comments: more core, better slotting, more even response, more consistent sound. Without a blind test, I do not believe anybody anymore.
Here is another site to check for detailed, scientific research:
Welcome at the pages of the IWK (Institute of Musical Acoustics)
then click on research, then mouthpiece forms, then variation of the bore.
The graphs are pretty much self explanatory. The german word for mouthpiece throat is "Bohrung" or "Seele" (translated:soul-I think a much better name).
What is immediately apparent is the change in intonation for almost every note on the trumpet. That is why some people perceive a darker tone - because it is sharpened when bored out and they have to lip it down which steals brilliance!
Please note this is a university research project with no commercial interest. It corresponds to my personal experiences VERY closely.
Moral of the story: do not bore out your mouthpiece unless you are sure that the deficiencies will be compensated for by that action (almost never the case!). There are a lot of players working much harder than necessary because of this easy to perform but not too intelligent "modification". The numbers do not lie - even if the player is strong enough to compensate. That energy would be better placed in making music and not fighting the equipment!