CCS, you're likely the exception, and why I made the point. Only a few measure before they put them in; I don't know for certain that the vehicle manufacturer does but at the speeds of production it doesn't make sense that they would. As long as you have a sharp edge on the tip and the ground strap, and enough voltage to jump the gap, there should not be any really noticeable difference in performance.
I will agree with your statement that 100% gap check at plant doesn't make sense but probably disagree with you that automotive assy has more requirements than what the outside world knows about. Maybe a new posted thread should be created vs. hijacking this thread. Previously, I was a process mfg engineer where our company designs products like brake transducers, occupant weight sensors, heavy duty fluid pressure sensors as part of the portfolio for the big Asian and European vehicles. Their necessary demands and requirements upon us goes into the demands we have with our car makes.
In this example, what are the top three topics about plugs generally discussed? Gap, torque, brand??? So if you were part of the Subaru engine assy team and plug installation was under your ownership...gap would be listed as a key characteristic. There is a pretty tight tolerance for this spec so doubt if a precursor check is used. That means by definition this aspect must be checked more than just shift start or time check vs. output because now your into 5 or even 6 sigma quality level in which the plant's goal is striving for. Let's say realistically <250 rejects per 1M opportunities that can escape and still be called a stable robust process.
I am familiar with all the checks and balances that went into mfg of sensors and all the requested automation that goes along with it. For spark plug gap, that measurement can be made a mile away at way faster than engine plant assy thruput speed. I know these plugs are purchased in bulk trays and not individually like we buy them and staged where the gap is exposed and correctly orientated so a vision camera inspects/rejects thus 100% inspection before presenting to technician or even a pick and place robot arm staging queue that will zips them into the engine.
So to get back on track for OP topic, I checked my notes and made a slight err in that I did replace the plugs earlier along with all the timing and original roller hardware at 95K because factory recommended age exceeded well before mileage. So for my application and my findings suggests that yes...my model is hard on spark plugs.