The Pro-Spec kit has the 2.25mm chassis, the floating servo mount, slightly different top deck and (supposedly) different material shock towers. All in all basically an R1 that's a tad more flexible. However, I believe you can make the R1 just as flexible if you leave out the top deck screws that screw into the motor mount.
I actually upgraded my R1 to the 2.25mm chassis just as an experiment (I get bored
), and so far I see no negative impact. I don't see much positive either, but the car handles fine.