Another thing to take into consideration as well is exactly what your definition of a "good" or "better" pattern is.
That of course, depends on your game. I have found that there are certain 3.5" 12 gauge guns that do pattern better with 3.5" shells than 3" shells. However, all of those guns, without exception, were overbored Mossbergs.... basically 10 gauge barrels (835s and 935s). And when it comes to turkeys.... you can't beat a 10 gauge.. its a like a deaf mute going for Miss America so to speak (or not.... get it!... sorry that was mean
).
When it comes to waterfowl patterns, I've never seen a really good shooting 3.5" gun pattern wise. Not when you look at the big picture anyway. Quite frequently, I'll get a good number of centrally dense pellets, but I'll also get a huge increase in the number of flyers. When you divide out the overall density... its almost always lower than that of 3" shells.... meaning that the pattern as a whole is really less effective.
Buckshot is a whole differnet story that really applys mostly to us southern gents. The thing with 3.5" buckshot, aside from ferocious recoil, is that those 18 pellets of OO are going much much slower than even a 3" load of OO. Most of the hardcore dog runners I know still shoot 9 pellets of OO out of 2 3/4" shells exclusively. Why... speed. A faster pellet of the same weight will give you better penetration and better ability to break bones, and with buckshot that is so very critical to success. A lot of these guys take shots that really should not be taken, and like a blind hog, they are successful probably half the time (thats optimistic). Plenty of bragging about the ones they shoot at 80 and 90 steps (of course, range guesstimation isn't one of their strong suits either). But you don't hear about the ones they bugger up. Generally, if you keep your shots under 50 yards and have patterned your gun and found a load and choke that works well.... it really is almost academic.