You mean the rooftop storage rack...or the rooftop solar panel racking...not the rooftop deck, right?
This will help with your insurance, because no insurer we've found will cover a vehicle with a rooftop deck. So, we knew what you meant!
As to your actual question, though, I'm surprised (but believe you) that composite weighs less than the wood. I've done decks (at ground level) and stairs with composite...the Trex brand...and it was quite heavy. I guess things have come a long ways since then. You're right about the intermediate supports, as it will sag more easily than wood. Over the long haul, the composite definitely holds up better in appearance.