Starkiller is late teens in the games and yet is ridiculously powerful
In fact, on this forum http://www.swtor.com/community/arch...p/t-790858.html
he is placed 11th strongest force wielder. However he is so young but yet so powerful. So, how powerful could he have become, I know Sidious believed that he would surpass him, but could he reach luke level?
He would've surpassed Vader at some point and assuming Sidious actually trained him in the ways of the Sith, there really is no telling how powerful he would've become
__________________ "Happiness is a lie. Life is horror. The light is always dying all across the universe. The last star will flicker out someday, when it does, all that remains is shadow. And I will be its king!"'-Amahl Farouk
The lead developer said he had the potential to become the most powerful Force user ever, so he could well have surpassed Sidious. I doubt he would have reached Luke-tier though.
__________________ "Happiness is a lie. Life is horror. The light is always dying all across the universe. The last star will flicker out someday, when it does, all that remains is shadow. And I will be its king!"'-Amahl Farouk
Well if we interpret the statement as era specific, it would only imply Marek could have become the most powerful Force user thus far, which would only put him above Sheev.
@Sinious, DE Palpatine is Luke-tier perhaps, but ROTJ Palpatine is really not.