Well, that kind of brings me back to my original point. I know a ton of people who "fry" their arms with direct arm work and make no progress - probably the majority of people in the gym. If there was some magic combination of arm exercises that were really effective people would have found them by now. I conclude that supperior stimulus is not in any combination of direct arm work although one can certainly add a bit in. All that being said, I don't know anyone using a proper program and consistently putting poundage on their squat, dead, row, and presses that doesn't gain arm mass regardless of whether they train it or not - and usually, if they do train them directly the workload is very minimal and maybe only an exercise or two rotated around (doesn't seem to matter at all which ones). This is for naturals as well as drugged lifters. So, supperior stimulus to me would be proper training in the compound lifts and adding capacity there as a first priority and then a very distant second worrying about direct arm work.
I just never see people pulling or squatting over 500 with shitty arms yet I see a ton of people who have never thought to use a power rack for anything other than a place to set their curl bar between supersets of 21s and dual decline hemoraging bufu curls make no progress for years - to be honest, this is pretty typical.