Hi!
SamD you are absolutely right, adding weight is no good .
You can do so much before adding something. You can take the Dremel drum sander and start sanding away most heavy structures in the fuselage...people doesn't seem to understand that most formers and other stuff is just not needed.
Use light foam wheels, light battery packs...why use heavy old fashion Nicad:s when there is lighter NiMh cells available, 950mA NiMh cells weight the same as old 250mAh Nicad pack. In a small model 300-400NiMh cells could be used and those weighs half of an old 250mah nicad pack.
Remove servos and place them as much forward you can, don't use standard servos 4-10g servos could be used as throttle serv and Hs 80-85 on elevator and aileron.
Use pull and pull fishing wires, thats the lightest possible stearing system...or 2mm carbon rods also good.
I never place any unnecessary weight in my airplanes, never! I rather rebuild the complete airplane than adding weight because I know from experience that adding weight is going to make the airplane a bad performer and going the easy route, adding weight is simply just ain't worth it...

Regards!
Jan K
Sweden