Feoh: About S80 and R grade stuff

[sbd2]

I have a big doubt about this: I have a Feoh Wizard, at lv 85 he had Vorpal Robe and the S80 Staff (I don't remember its name). When I hit 86 I bought Immortal using Diplomas, but when I put my new armor on, M.atk decreased, also the S80 staff has more M.atk than R staff (277 against 270).

What am I missing ?

And, another question, I saw a lot of threads talking about if 2 hand staff is better/worse than 1 hand staff + sigil. I would want to know if the +5% M.atk that Superior Sigil Mastery gives is 5% of weapon M.atk or 5% of total M.atk.

Thanks in advance.