Ive only completed the quest on my lvl 47 (56 archery) and the damage was like 20, is this normal? and can i increase it a lot or not? I would hate to waste such a unique bow

Yes I think this is normal because my level 55 character (80 archery) has only about 36 base damage. I think the lack in raw attack is made up with the secondary effects (which I think are AWESOME)

yeah that is pretty awesome, can you upgrade it? because my lousy 30 damage crossbow shot up to 50+ after upgrading it

As a level 47 with 100 archery, all the damage perks, legendary smithing upgrade, and a circlet that increases archery by 40%, it does around 112 damage for me.

I'm starting to wonder myself, is Auriel's bow a leveled item or is it static? 27 Damage seems a little weak for a bow of such legend, although I only have 40 archery I have found that ebony bows far outclass it in raw damage.