Welcome to the forum and posting.
I just walked out of a well known gun smithing and choke builder today in Can. He had told me that Yes a shot pattern can be Over choked and most times it happens with the larger Shot size 4,5's. Most times 6's will pattern better because of the constriction sizes in the chokes. A choke that is to tight will deform the shot cause fly offs with a wider pattern. Fitting your turkey gun with the right size choke plays a major factor in a better pattern. The .665 dia. chokes are sized for the 6 shot pellet size, so a choke that is tighter may not shoot a tighter pattern, but just the opposite. Depending on the size of shot you would want to shoot,may depend on the size of constriction. I like to shoot the larger shot my self and have found that the .665 works for me the best....BT