It's lighter, but in my mind, a couple hundred pounds of weight (give or take, I forget) is nullified by the other advantages. Unless said RSTi has the dash swap as well, the interior of the newer cars is much, much better. The chassis is stiffer by ridiculous amounts. And of course, "newer" means fewer issues: less rust, less bad paint, etc. etc.
Not to mention that the newer ones are safer, as well.
I love RS swaps with all of my soul, but if I had the money for a spankin' new STI, I wouldn't hesitate. That in mind, I'm still saving up a few grand for a WRX motor swap. $3-5K or so, to me, sings a different tune than $8K+ plus the transmission, brakes, differentials...........................
Which is precisely why I can't wait to read about the '08 Imprezas, with track tests. Never mind the new multilink rear suspension, but that car will probably annihilate all previous Imprezas, stock for stock.