I've had limited experience in using spreader loads in a variety of guns, but I do use them exclusively in one of my guns.
I have a 16 gauge BLE, which is choked improved cylinder and full. It is my fulltime woodcock and grouse gun. I use 2 1/2" RST spreader loads in 7/8 ounce.
What I have found in my gun, is that they don't substantially open the pattern. In my IC barrel, they open it to about C, and in my Full barrel they open it to about IM. The increased spread was marginal.
However, what I did find is that by using the spreader loads it substantially improved the uniformness of my pattern. When I use a standard RST load (and shotcup) the pattern is more center dense with more open voids as it filled out the cylinder pattern circle. With the spreader the pattern is much more uniform out to the pattern circle with fewer voids.
Your gun may vary.
