Let us wait for Blackbeard vs Shanks to have actual answer.
I think this isn't a proper way to gauge it.
Blackbeard is the peak of DF merchants.
While Shanks is a hakiman, he is not the peak.
And there is always the narrative that has certain tasks to accomplish.
Oda would either have to make this a legit win for BB or offscreen it for DF merchant to beat a hakiman.
If Shanks parallels Brook, Blackbeard could wipe out entire RHPs paralleling the demise of Rumbar Pirates.
The closest to peak DF merchant vs peak hakiman we will get, IMO, is Zoro vs Shiryu.
Shiryu will be peak DF merchant alongside Blackbeard, potentially even above if his Awakening is nothingness.
Zoro wont be peak hakiman during that fight yet but he will be as close as it gets to it.