I'm not sure that kind of math works here. Top tiers tend to do better against tough foes when fighting on their own anyway. With Skyfathers, anything can happen since the pretty much can have any power the writer wants them to have.

For the record, I think current Silver Surfer is in the same league as Thanos. I realize that continuity implies otherwise but I'm of the opinion that Surfer was watered down after years of showing up in team books.