I have never messed with the 3.5" 12 gauge much and especially haven't since the "newer" 2 Oz HV loads came out even though its been 20+ years ago now that they came around. Do the 2 Oz loads seem to pattern as good as or possibly better than the original 2 1/4 Oz loads? Do the 2oz in 3.5" pattern much better than a 3" with 1 3/4-2 Oz of shot?
I actually have no need for a 3.5" load but want to play with the new to me Super X2. I am sitting on nearly 130 3" mags of various flavors.
I do have a XXX Comp n Choke, original Star Dot and a .680 Hastings. All of these have done pretty good with the Hastings doing best with 4s.
Don't know what gun you're using, but my 3.5" mossy really likes the 2.25 oz longbeards through the original star dot choke.
To your question.
The 2 ounce can pattern just as good as 2 1/4 ounce loads in 3.5 inch. In some guns they will pattern better, other guns prefer the heavier loading.
2nd question, A 3.5 inch shell pushing 2 ounces will shoot better than a 3 inch shell pushing 2 ounces or 1 3/4 ounces. ( assuming apples to apples comparison).
Not tremendous when you consider recoil is brutal but you can certainly get a denser pattern and couple/ few more yards with 3.5 inch.
This applies to lead shot .
My SXP shot longbeard 2 ounce loads better than 2 1/8. Carlsons LBXR .665.