Notice how the full choke threw a larger more open pattern than the improved modified. Guess you just never know til you try a choke and shell combo. I've read and been told the little 28ga doesn't tolerate a full or extra full choke well. Of course it depends on the load and shot size.
Also I was in academy earlier today and noticed they had some yildiz 20ga full flush fit chokes marked mobile choke on clearance for 5.99 a piece. For those of you that shoot the 20ga o/u and live by an academy you might have them discounted there as well if you could use one.