I have fired about 50 shots through mine. Basically the little time I do have with my 8.5 I did not find this to be a problem as I am 6' 3" with monkey arms and Jack Skellington hands.
Now that I am aware of this potential problem (the other thing to be weary of is muzzle direction with it being so short) though I will definitely be watching out for myself and other when this 8.5 is in use. It is easy to forget something as little as this.
I do not think I will be doing this mod though. My .02 cents (with ain't worth crap)is I believe education, awareness and common sense come into play here. That is not to say that this mod should not be considered by others.
Taking in concerns what you said, everything works as long as you shoot your shotgun in non-stressful situations, like target shooting or hunting.
When you use it for self-defense like against predator animals or intruders, "education, awareness and commons sense" goes right out the window. Trust me.
All your "fine motor-skill" fly-by and most of the time, you have hard time to even dial 911 because of the adrenalin pumping through your veins.
Your parasympathetic system takes control of your brain to SURVIVE and then... You forget about where your thumb is resting and you fire...
Just be careful not to look like this:






























... (the picture shows a .500S&W just for comparison)
























