By feel, I'd say the FG fender and original fenders way about the same weight as is and filling in the vents wont help much with lightness for sure.
The real idea behind these FG fenders is to replace my rusted metal fenders with something that wont rust again in a few years. My DS side fender doesn't have any visible rust on it right now so that one may stay on the car for a while, but the PS fender that I'm replacing now has some really bad rust on the rocker panel area and some other major paint chips.
Hatch Nation #13 : Rally Revolution #13 : ROTM Jan '13
It's Bryan, not Brian
RSVT-170 Build in Progress
Derusting Rusty Build in Progress