How powerful is Galen Marek (potential wise) exactly?

Hayden Blackman compared him, in potential, to Luke. Given that Sidious heavily implies that Marek could have surpassed him, I'd say he's not too far off the mark.

Yeah i'd put him around luke/caedus potential

Isn't there a statement out there where it basically says for the purposes of "The Force Unleashed" Galen Marek is given Force Potential that surpasses all other established characters? I swear I've heard this from someone before, but I'd have to look it up.

Nah, I haven't been able to find it. I guess someone was just pulling a quote out of their ass, wherever I read it. 🙁

Still though, I'd say his potential is pretty haxed, even if he didn't ever really surpass the likes of Vader.