I happened to swap the cap from my A6 dual coil (dual-opposing holes) onto my ST MiniRDA. I had previously opened the hole on the ST MiniRDA to 1/16". It worked well, but the draw was still a little tight. With the dual hole cap, the draw is easier and I experience even more vapor.
I say all this to ensire you that some of these, if not most, rebuildables have holes that are just too small to begin with. I appreciate being able to open the hole to my preference. I say if the draw seems somewhat tight and there seems like there's room for improvement, it's perfectly fine to open the hole to suit your desire.
I have read a recent report that the FDA is going to be regulating the size of the airhole to prevent inadvertent use by people who like vapor in quantities that are unacceptable to uneducated non-vapers, so drill away before it's too late!