Here is the honest truth:
When people start talking about what beats what, and my buddy beat this or that, its nonesense, because jetting, gearing, and other factors are so varied that its insane to compare and make claims, unless you have some equalization, or at least some knowledge for sure about both machines.
I own both, both are modded. The Z has both Hot Cams, 400ex carb, LRD pipe, and a DRZ gasket (12:1 comp). THe 450R has HRD and a LRD pipe.
Now, my wife, is 90#, its her Z. That thing will pull the hill about 1/2 mph faster up the big hill than my 450R, with me on the 450R, weighing 230#. On the flats, she can pull me. Z's do VERY well on the flats. They are not as good up hill.
Piped filter Z would do pretty well against an out of the box 450R. Not HRC 450R.
Don't make comments you cannot back up, gnarly......
I have the firts Z that came out, and it supposedly has the weak axle, frame and the rest. No problems at all, granted my wife rides it mostly, but there was 6 months that I was quadless (waitng for Honda), and I rode the piss out of it, flat landed, jumped BIG, oh yeah I have wheel spacers on the back, nothing wrong with it at all. Its not what everybody says. Take care of your stuff, don't beat it, don't run into trees, and it will be fine.