I think Roman has pretty much maxed out Lamar's NFL potential. Putting Lamar in any kind of predominantly passing offence strikes me as a recipe for disaster.
I agree that it's reasonable to conclude that Lamar's passing stats would be better if he had better receivers. I mean Josh Allen's stats would probably be better if he had Mahomes' and Burrow's weapons. In evaluating Lamar as a passer you have to ask not only whether he would benefit from having better WRs but also whether he has the ability to make the WRs better than they would otherwise be. I don't think he does. He can't do what Josh Allen do.