In my opinion there is a caveat with 3.5s. That is, that in order to maximize the increased payloads offered by the longer shells, you need the over bored barrels like the Mossbergs. I have yet to see a standard bore sized 12 guage pattern the 3.5s as well as the big Mossbergs.
Here's a pattern with skinny barrled SBE 2 with 3.5" Nitro's.
Using custom loads isn't really a comparison of averages.
But out of curiosity did you count the number of pellets in the ten?
what size show at how big of a charge