Getting the chassis alone could be a very good upgrade. The suspension mounts greatly lighten up the car though. Its also more consistent run to run because they don't build up slop as you drive more.
I noticed a huge difference going from a stock 13 to a full conversion rx4 (although I have the v1 chassis). My car gained so much steering and thus far, I've put down some of the fastest laps in stock sedan at my local track. The top guy here did an 11.1 (he was in the A-main at 'birds in mod and got 2nd in the A 13.5) while I was at an 11.3. I'd never been that close to him before and I believe it's because I finally got a dialed setup.