Yep, unsprung weight. But unless you're racing (or obsessed with acceleration) it's not a big deal - especially since you're happy with them. I really notice a difference in acceleration - and, to a lesser extent, braking - with my original DDS rims versus my genuine steel Porsche 356 rims, but I like the look of the 356 rims so it's no big deal to me (on the street).
And the above post is correct, the original Fuchs are forged aluminum (not magnesium).
Speaking of magnesium, polished aluminum will always give a brighter (sustained) shine versus magnesium, because mag starts to dull the moment you move on to the next wheel (after polishing). That's why magnesium wheel owners either have no skin left on their knuckles, or they are just let them do what the wheels want to do anyway and call it patina.
