I would say that what Keymaster suggested sounds true to me. Unless you use sub-pixel "OFF" (unchecked) or high amount of material subdivs on metals in Vray, you will not keep the same energy to reflectance as there should be because it will clamp. Most people over-compensate this.
Other thing is, that glossy levels range [0-1] don't correspond at all with Vray, and neither the texture driving amounts to same result. I don't think it's better or worse just different, but definitely needs some using to, preferably creating most crucial materials from scratch. I only keep converted mats for non-important things like decor.