I've shot 100s of patterns with various 12 ga and steel loads. 30 inch circle at 40 yards. I have never seen a 3.5 inch shell stand out head and shoulders above a good 3 inch load that shotgun and choke combo like. Usually there similar to that same shell in 3 inch, more blotchy, less even and often less pellets in the circle. Not worth a dime more than a 3 inch.
I've also patterned lots of 2.75 steel loads, score and challenger, that pattern very very well. Put just as many pellets in the pattern as the longer shells. Just because a longer shell holds a few more pellets does not mean they arrive on target. There are plenty of aftermarket choke companies that say the 3 inch will generally outpattern the 3.5 inch. Its got something to do with a square shot charge patterning better.
Speed wise generally the 1400 or 1450 fps shells will outpattern there 1550 counterparts in the same line. But not always. Depends on the wad used. 1400 fps bbs that pattern well will kill geese out to 45+ yards. If you can put that pattern on the bird.