If ya decide to change the FCG bits,.......get an extra "c clip" when you order parts.
those things have a nasty habit of jumpin of the bench. Be sure to have a good of snap ring plyers
You are correct in your line of thinking........ProMag pistol grip and Benelli C stock
Personally, I chose to change the internals. Installing the Geissele hammer and FFT trigger was a wonderful addition!
This is what you want, you just don't know it yet.
http://forums.benelliusa.com/showthread.php/32971-BENELLI-OPTIC-RAIL-The-decisive-Trijicon-RMR-mount-for-the-Benelli-m4
I can not justify spending an extra $100 to save 5 ounces. Shooting light weight shotguns sucks.
If ya really feel the need to shave off five ounces, i would suggest a bit of gym time.
When you reinstall..........use no thread locker. A strap wrench is all I used for my Benelli tube install.
It has not loosened, I placed a small, black witness mark on receiver/tube as a visual.