The first shot is the most important , hit with the first shot first before you try for more shots.
It will work for close rang ducks but if your 870 is a 12 gau. I would stick with it .
Yes the stock will get scratched up
Some like the strait stock , that's up to you
Yes you can shoot 2 shots faster ,but hit with the first and you don't need to shoot the second.
Who cares how much "BETTER" your gun is ,it's how good you shoot it.
A 18" lead at 50 yds ? on a crossing target at 50 yds that is flying at 20 mph you will need over 5' of lead