I have both the "Squid" and the "Phoenix V4" from hoosier, both in SS, they are basically the same aside from a few differences. Anyway, I drilled out the airhole on both to 3mm. I like an airy draw, so that you can either do a slow controlled draw or a few super quick draws and you get great performance either way. I also position my
coils as close to the airhole as possible, so that it acts sort of like a drag and adds a bit of resistance to the draw, since the coil + wick are sort of plugging the air hole a bit due to their close proximity to the air hole.
This really only works well with sub-ohm coils, at higher resistance the coil takes so long to heat up and the vapor produced is thinner to the point where it's incredibly difficult to get a good vape (IMO of course)
The opposite also seems to be true that smaller holes don't work well with really low resistance coils. Again, just my opinion, but the smaller (stock) air holes don't give enough airflow to properly take advantage of the coil at those power levels. After drilling mine out vapor production seemed to increase by a noticeable amount and the vape got a bit smoother, still great TH but just a tad cooler, and more of a feeling of billowy clouds going into your lungs instead of hot steam, if that comparison makes any sense.
From what I can tell most people drill theirs out to around the 2mm mark. I'd start small and work your way up as far as size goes. You can always make it bigger, but making it smaller will usually be a hassle involving stuffing stuff into the hole to restrict the airflow a bit.