If that's the case you'd better just give up on pipesmoking altogether.
Across 50+ pipes, Ashton, Ferndown, Big Ben, Blakemar, Nording, Peder Jeppesen, Peterson, Rossi, Rattray's, Radice, Ser Jacopo, Savinelli, Tsuge, and then an assortment of basket pipes, all of them are drilled to a 3mm draft in the stem.
In terms of air flow they're all virtually identical, which is to say it's pretty poor compared to a Missouri Meerschaum 80 cent plastic stem.
Unfortunately while Brebbia and Vauen do use a 3mm draft through most of the stem, they like to finish the button with a single 2mm hole only the height of the button slot, which is pretty bad, but you only need to slightly extend the slot to restore the industry standard 3mm airflow.
Beyond the most basic drilling with a 3mm draft intersecting with the button slot, there are three exceptions I've found.
First is an old "Icarus" brand bent egg from Briarworks, the drilling on that pipe is truly impressive (very deep slot) but from the glimpses I've seen of clear stems on recent Briarworks pipes it seems they've given up on the style of stem with an extended slot.
The draft on the old Icarus is only slightly larger than standard at 1/8" or 3.125mm, which doesn't seem like much, but when virtually every other pipe in existence is 3.0mm it's noticeable, plus the extended slot significantly amplifies the effect.
Next just recently I've found a Viprati Lumberman and "S. Klein Design" from Scott's Pipes that were also drilled to 1/8" (3.125mm) instead of the standard 3mm.
The S. Klein Design pipe in particular has an extra deep slot like the old Briarworks Icarus. The stem on the Viprati is a little rough, clearly "hand made", but at least some extra effort was taken to slightly improve airflow.
I will say here that both of these pipes are Lumberman shapes, and that means the stem is really short and that makes drilling really easy, I have no idea if bent shapes or longer stems from either brand take the same steps.
We all know Castello is renowned for maintaining very open airflow, but that reputation has inflated their prices to the point where you're paying 2-3x more for the simple luxury of a pipe made with a slightly larger drill bit.
It's really astounding that there is so little variation across the entire pipemaking industry.