Consider the damage Batman can do. With prep, he's taken down foes far more powerful than he is (not all PIS). While SS ain't no Batman, he doesn't need to be: the vast array of powers at his disposal more than compensate.
Think of it this way. Suppose the shoe were on the other foot: Surfer's hangin' with the Earth Team, while Dr. Strange's the one planning a first-strike attack. What chance would the Team have then?
Another thing: let's suppose Strange does legitimately possess abstract level limits to his powers. How often does he go there? AFAIK, not very. His average power level is much further down, much closer to human.
Surfer, on the other hand, even on average, operates on a much higher power level, even if his limit does not (?) exceed Strange's. Overall, Surfer is the mightier of the two, except in those instances were Strange has the time to prepare and reach for those levels beyond the Surfer's power.
He doesn't get that opportunity here.