I have a 12 ga. Sporter and while that is true for the 12, it is not for the 20. The reviews I've read state the choke constrictions for the twenty are close to what is stamped on the choke. But as with anything patterning will show you what is really taking place. If full is too tight, You could just purchase another Mod and go Mod/Mod in a 20.Not sure about the 20 ga investor DS. It the 12 gauge are more open than normal so I would highly recommend patterning them with the shells you are using. Give yourself the best chance at being successful.
I'm with Oscar on this all the way. I like #5 copper plated in an open choked gun. I can see possibly choking as tight as modified with #7-1/2 but I really like the way #5 patterns out of my skeet and IC chokes. And I like the way #5 drops birds.I have used skeet and skeet for years. If I need something tighter, I use skeet and modified. Most people use way too much choke.