If an Over-powered class, to you, has no counter, then 1 example of a counter to a class makes it no longer overpowered; and that's not ok in a game with such varying combat flexability.
I think you might find a better definition of Overpowered being a class (or soul in this case) that when compared to other souls performing the same function, it is clearly superior.
i.e. What Mage Spec should I be when PvPing at 50? If Pyro is your only answer with anything else laughably behind it, then there's an issue.
While I am tired of hearing about Pyros, I can tell you this much.
Does a Pyro have a lot of burst damage? Yes
Does a Pyro die very quickly when not in Ground of Strength? Yes
Does a Pyro have tools that other Mages must have in order to compete in PvP (other than GoS) ? No
If Ground of Strength was given to any other class, would it be OP? Yes
though my synopsis is woefully without numbers or detailed comparisons, it's clear what the culprit is in this instance.
There are a lot more problems in PvP than mage burst, that's just a symptom in the grandiose flu that's PvP.