effective....
If you had a police style folding stock that would allow he gun to still be shoulder fired it would certainly make it a better weapon IMHO
The only advantage to a pistol grip is reducing the overall length of the weapon, in conjunction with a 18" barrel for easy concealment in the home, and some people would argue it makes for easier movement through close quarters.
I have one on my 870, I have never fired the gun with it on, but I am pretty sure it will not be a pleasant experience.