I think its easy to assume current yonko Shanks is way ahead in power than when he had duels with Mihawk (with 2 arms). So is Mihawk of course, they have way more experience and mastery of haki than before.
The way Shanks has been portrayed, his power comes from his use of haki. He could have one, two or three arms it wouldnt matter more compared to his haki, which is the power that differenciates real powerful people in the OP world.
Both Shanks and Mihawk had less haki mastery 10 or whatever years ago, so they are currently stronger than their previous versions no doubt, it doesnt matter if you think one is stronger than the other, they are both stronger now than in the past.