 First Roger vs WB:

Despite his title, the most destructive DF and inhuman physique, WB’s greatest feat comes from his comparison with Roger.

In the panel where they clashed, WB had no idea that Roger was terminally ill. They went on fighting each other for 3 days straight. So, even sick Roger was able to fight equally with WB. 

Now Shanks vs Mihawk:

We know that they fought duels in the past and there was no winner. 

That means, even with his sword mastery and top 1 black blade, Mihawk was equally matched with Shanks. So the title kinda becomes irrelevant the same as WB’s. 

The most interesting aspect of this rivalry:

Mihawk doesn’t want to fight Shanks anymore because Shanks lost his arm. If WB had known prior that Roger was sick and has few days left, he would never have fought Roger. But we got to see, even without that aspect, Roger was evenly matched with WB.

So what gives you the idea that if Shanks and Mihawk were to fight today, Mihawk wins? Title alone can’t carry Mihawk agenda anymore while everything in the story states otherwise.

Conclusion: if you believe Roger >= WB, then the same scaling also applies for Shanks and Mihawk. Titles are just Oda’s way of telling how strong a particular character is while he glazes tf out of his Straw Hat holders. 

Roger - Whitebeard rivalry confirms Shanks >= Mihawk.