I think there is a tremendous amount of marketing hype designed to sell 3 1/2" magnum turkey guns going on.
All that is needed to kill a turkey is to over-saturate the head/neck with shot.
That being said, a 1 1/8 oz #8 trap load out of a full choke will kill a turkey over decoys deader then Al Capone, every time.
Personally, I find a 2 3/4" 1 5/8 oz #5 Mag all the heavy recoil I care to subject myself too.
If you can get within 40 yards, a full choke is plenty if your gun patterns them well.
Depending on the gun and shell, the X-Full & XX-full chokes may not pattern as well as the standard full choke tube you already have.
You just need to get the loads you want to use & pattern test your gun with them to find your effective range.
rcmodel