Don't underestimate wind chill, I did, and had to bail on my bike trip there, was going to bike around it. Froze my a$$ off after only 5km max out of Reykjavik had all my normal 3 season gear on at that point and hadn't even hit real cold yet, and that was the south. That was July if I remember right. Was a long time ago, but I do remember blue skies around me as I turned sadly back and gave up.
I agree, bring 20 degree bag, bring real wind/rain gear, it's going to be windy and it's going to rain, that's almost 100% certain.
One thing the temps don't tell you is how bone chilling cold it gets that far north when the sun goes behind a cloud, the air holds no warmth at all.
I also remember reading that a lot of the streams have a very fine volcanic/glacier melt dust in them that will totally clog filters, so I'd check that with someone who has been there and used filters, can't say that from first hand experience though, but don't overlook that part.