A good fiberglass front clip often weighs less than fakie-looks only carbon fiber front klip stuff, albeit at a similar price. The front fenders are engineered to add stiffness as steel units. You will probably lose some stiffness in your front end, unless the fiberglass fenders are engineered to be stiff (they should use a honeycomb core if you are looking for strenght charachteristics).
On the engineering tip, yea. The rear swingaxle in a FRD-like a VW- is not going to be improved upon in the aftermarket. Anything that would work better than the stock setup would cost way more than it is worth. On a RWD, yes, many systems are available that are far superior to stock. Their benifits lie solely in the fact that they strenghten the rear sub-assembly of the chassis to accomodate more power, so the engineering skill is low to provide a quality piece. (Hence a Ford 9", it's just tried and true, you follow the basic design that has proved sucessfull.
Black Lotus' reply says it all. DO NOT ADD WEIGHT!!!. All this will do is slow the rotation of your vehicle. Relocating your battery is #1 mod. What I have tried but never seen is the relocation of the radiator to the rear. With the appropriate routing and ducting, this puts more weight in the rear (below the trunk) than the battery, and can also be used to add downforce in the rear if ducted correctly (intake in the bottom, exit in the low pressure area of the tail of the car-the reaon for wings in the first place-.