I wouldn't use anything more restricted than modified and doubt more restriction is needed. If you think you do, you better be sure the shotgun barrel you shoot the ammo (shot) with is certified to handle such. Steel can cause damage on too much restricted barrel, if it's not manufactured to withstand the added pressure. As stated, steel and mony other non-toxic shot typesdon't compress like lead did.

For decoy hunting I think IC is actually a better choice, better patterning choke. Even modified at close range with a hit to the body can turn a bird into a chewed up mess.