Schwab,
I assume that the mouthpiece builders (Kanstul seems to only be a mouthpiece copier - I see no basic R+D on their part. I do not know much about Warburton, but GR Gary Radtke has the math down!) know what they are doing and that there is a specific reason to have the backbore/throat/cup/horn balanced. The modification of "opening the throat" does destroy that balance (when we run a drill through the throat, we make the throat longer too, adjusting the backbore AND throat can keep the length more "optimal"). This does NOT mean that a large throat is bad. It means that more has to happen than just running a drill through it.
As far as the pros go, only opening the throat makes the slots fuzzier, and that makes playing a marginally in tune professional C trumpet MUCH easier. As the orchestral standard has up until recently been these marginally in tune C trumpets, it is clear that an open throat was maybe even necessary. With the current generation of much better in tune horns, a smaller throat would increase the efficiency of the entire system though. I have seen no research on whether the "pros" just run a drill through the mouthpiece, or in fact have the mouthpiece "rebalanced" with a larger throat (and backbore). There is a significant difference!
Opening up a 3C by yourself will not improve anything. If you need a more free blowing mouthpiece, the backbore AND throat need to be adjusted.
The best research that I have found on this can be found here:
Welcome at the pages of the IWK (Institute of Musical Acoustics)
click on research, then mouthpiece forms. If you go down to the part on throat (called seele in German), you see very clearly how opening the throat alone only screws up the intonation. This is worth a read before pulling out the drill!
The basic calculations are physics and to get a good mouthpiece, a balance must be struck. There may be valid reasons to deviate (bad intonation of the horn for instance). It is better to think about what you need and then to research how to get there instead of just drilling out a mouthpiece and after a couple of months of ignoring the truth, finding out that it was a mistake. Yamaha is making waves with their "in tune" C trumpets though - and that will lead to changes in mouthpiece design.