The difference in weight for the fg fenders and ccf fenders would be pretty minimal...
But I really dont think most people are looking for a full underbelly pan. Besides it being super expensive to make and realllly expensive to ship from what Ive read in the windtunnel, underpans can be somewhat unsafe unless you have done alot of other modifications to the car aerodynamically. (race car- and then they would have made their own to fit their own needs)
As for the front splitter, I was able to make a set of cheap turnbuckles to support them from parts from lowes and a machine shop. Either you could make them yourself or leave it to the buyer to do it. You could also support it by bolting it to the metal strapping rather than the soft urethane. http://forums.hybridz.org/showthread.php?t=132254