Close to Roger (pushes him to extreme diff). I guess equal to current Shanks, with 2 armed Shanks being ~ Roger. Yes, I believe the difference between 1 armed and 2 armed Shanks shouldn't be big, 1) because the main source of Shanks' power is Haki and he is a formidable swordsman in any case, also having adapted to his one arm while fighting his way through the new world for years and establishing his position.