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 .