It might be less than full protection vs. evil, yeah. I never number crunched to get the exact amount of dam redux. It does stack with normal "protecction", and with "mantle of the phoenix". "Mantle of the phoenix" does not stack with normal protection.
I do know, however, that "favor of the sun" also grants either resist negative or resist mental (or potentially other things, though I only ever saw those two). I tried using resist negative and resist mental preps when affected by favor of the sun and got the "You're already protected" message. But only ever one of them at a time.