Strange ... guess Champion has something to lift the atk cap then? Not sure what the UI number changes for Champ, but not for Avatar.
Before (buffless)
Total ATK: 2051 Worn/Spell ATK (Cap 250): 560 Server Used ATK: 1858
After (w/ Avatar only)
Total ATK: 2051 Worn/Spell ATK (Cap 250): 660 Server Used ATK: 2058
Avatar is obviously there because the Worn/Spell goes up by 100, but then the in game UI never adjusts along with the Total ATK, but then the Server Used ATK goes up by 200? Confusing to say the least.
To clarify then ... is it beneficial to keep casting Avatar on myself when there is no Shaman present?