How to gauge the power of Assume Supernatural Ability?

[MaxiDuRaritry]

Sirine's Grace (SpC) is a Bard 4 / Druid 5 spell that gives you a bonus to Dexterity & Charisma, plus you get to add your Charisma as a Deflection bonus to AC, plus a few other things, but only for 1 round/level.

So 4th level seems reasonable for a longer-duration Unearthly Grace spell-equivalent.

Sirine's grace gives a lot more than unearthly grace, and Assume Supernatural Ability requires a lot more resources than a single casting of sirine's grace does. I think 3rd level should be fine.