of course.. but foam not so heavy..
spot welds also minimal in terms of weight.. but your car will feel stiff as hell.. a friend's subaru sti after spot welding was really stiff.. can feel the difference before and after.
Thinking about doing it b4 for the whole car... but thinking of carrying passengers once in a while.. not worth it.. I only did a bit on the front end at the crossmember area.. the rear is left alone, although I'm thinking of it once I finish my B20B. I'm using only stock bushes and a suspension that's not the pillowball type, but people riding think it's pillowball.. :P The only thing different is that there's not much squeaks or noises, apart from the engine moving on it's mounts during on/off throttle.
The R bushes that came with the knuckles really stiff... after 30K kms.. there's still no noticable play (although the knuckles are second hand) and there's no alignment issues, apart from the rear driver side camber, which has something to do with the chassis itself, not the knuckles, bushes or the type R upper link. Hopefully I'll get this fixed when I get around to spot welding the rear end.