If I want 3" load density out of my 3.5" loads I will open the choke up a size or 2
I do pattern all loads, guns and chokes I shoot, and I have patterned thousands of different load / gun / choke combinations with all of the guns I have owned and the chokes I have been thru. That is why I know that my 2 10ga's do not pattern most loads any better than my 12's out to 50 yds (I have no need to test past that). I have tried all of the Hevi-crap and fancy square this and that shot but I always find that plain old steel does the job just as good (dead is dead, right?) as ammo that cost 4 times as much. So if you want to talk about drinking the Kool-Aid or falling for the marketing hype it is right there in all the fancy high dollar loads not in 3.5" steel shot.
Granted, you can kill anything in the proper range with a 410 loaded with #7 steel (done it) but again I like the extra "pattern density" I get with the 3.5" loads. It is not about the velocity, it is about more BB's on the target. The 3.5 definitely with no question puts more shot in the bird than the 3" in my testing every single time. And of course the 3" puts more on target than the 2-3/4" Provided you do you part and match your choke to your load and game / distance.
Sure I shoot the fancy stuff when I get boxes of it for free or next to it at auctions or estate sales, and I find that some of the federal hi density loads work great when I am pushing 60+ yards on giant geese that have no intention of landing.
Bottom line is that the 3" and 3.5" are not necessary at all but each is a little better than the size smaller.
I will confess, I shoot black cloud but by the case at Dunns it is cheap and patterns great in 2 of my guns, (one of my sp10's and my Benelli performance shop waterfowl). funny how a load will work in one gun but performs like crap in its twin