I've heard the arguement before that the springs are made to tuner's specifications. But what I wonder is what are their specifications? I mean, Eibach & H&R springs are already made for aggressive driving. I wonder if you had an unmarked Brabus spring for an E320 and an unmarked H&R spring (remember, they make them for Brabus), could anyone really tell the difference? And I don't mean just the looks, I mean if you put them both under tests. I doubt there would be any difference.
Like Clauser said, you can compare it to wheels. My OZ Superturismos cost less than the Carlsson 1/16 RS, but guess what, they are identical, minus the different labels on the center caps.