XR - if you can wait an indeterminate amount of time for me to find the time to finally finish *cough* do *cough* my Master's thesis (assuming my other credits haven't expired, of course), I already have preliminary approval to study this very issue...though looking at holes bored in joists not studs, but same difference.
One issue I've found in my informal literature research is that very few people have really looked at this from a research/theoretical perspective. All of the code requirements have been around since the early 20th century (I think the earliest reference I've found so far that is identical to current code provisions was 1920s) with no real reference to hard data - just a "we've found this works well" sort of justification if anything.
Without any hard data to back it up, I would reinforce it. My limit is usually 1-1/2" to let rely on plywood reinforcing for a handful of joists. For this, I'd only let them bore this hole in a few studs - maybe 4 - and then I'd double the studs and add the reinforcing. That's just a judgement call - no data to back it up.