I have both Gen IV's and you can tell the difference. It's not a bad thing, I just think the wagon's springs are too soft. It makes it feel heavier.
As for performance, you really can't tell by the seat of your pants which one is faster.
As for the extra space, it's great. We just got back from a road trip from MD to Orlando and everything fit in the rear below the top of the back seat. We had two suitcases, a full size baby stroller, a box of snacks, a box with our cameras, a couple of pillows and some other junk. Had we taken the sedan, we would have had to get a roof top cargo carrier. It's also nice to have a roof rack. We drove down with some family and with the extra jump seat in the back we could go the parks in one car and save on parking. I just bungee'd the stroller the roof and off we went.
And as far as comparing it to the Caprice, like JJ57 is saying, there are two different cars. I can say however that I never felt under powered rolling down the interstate fully loaded.