Look at Zack in the upper left had corner of this forum. He is a '77 280Z with the Motorsport Auto Aero II kit on him. Crash bumpers are gone along with about 8" in length and 30 lbs of weight. Won't survive a 1 mph impact though without scattering fiberglass all over. Beauty has its price.
I also have this motorsport body kit on my 78 280Z. Pretty good quality and resonably strong. You can save alot of weight, the front bumper and shocks comes out dropping about 40lbs on the front as well as the lower valence (that doesn't weigh much). Same in the rear although the shocks are harder to remove without dropping the gas tank, but in order to maintain balance I removed the rear shocks. I ended up dropping 70lbs.
I need a new front airdam though, I had a bobcat jump in front of me on the interstate, I was cruising about 85mph then wham. The only reason I told you that was to say that the damage was not as bad as I would have thought. It just broke the bottom corner of the passenger side and could be repaired if I can't find another one somewhere locally.
To chime in with the others, the motorsport kits are nice. I only used the front piece... For the back, I took out the shocks and bumpers and filled in the holes. It looks VERY nice tucked under. NO protection though for the gas tank in case of an accident, I need to switch to a fuel cell!
As for the front, the air dam looks great, and definately helps at 100+ mph with the front end getting light. However, a buddy's girlfriend (driving a 300) BUMPED it at about 1 mile per hour and its now in 1000's of pieces.