In reply to Siward: Unsurprisingly the answer is 'it depends'
I don't think carbon tech has come on a massive amount in that time, unlike marketing hype! The trickle down IMO has been a lot more in componentry: 10 speed, unexposed brake and shifting cables etc, some of the technology from 2008 Dura-ace is now present in Tiagra for example.
Whereas some of the lightest frames were just appearing in the period you mention and they have not got significantly lighter since (if at all).
My experience: I have an ex-Pro Tour 2008 Scott Addict f + f built up with SRAM Red. In it's day it was the lightest production frame (780 grams) and was ridden by Saunier Duval Prodir (Riccardo Ricco, David Millar, Iban Mayo and others) and then by HTC Columbia (Cav etc) before they went to Specialized, current top end frames are not much lighter (in fact most are not) and it was very, very expensive.
I picked up the F + F for not too much and it had barely been ridden. It is an incredible ride, climbs brilliantly, faultless downhill, corners on rails, stiff and responsive, yet really comfortable. I regularly ride 100 miles plus on it. These are all the things you'll see said about today's top end bikes. Compared to my other up-to-date but cheaper carbon bike it is far superior. A top end bike from a few years ago was always an amazing ride and still will be.
However, here's the catch! I would certainly do the same thing again as having ridden a top end bike I can't see how I can go back to something middle of the road, it just won't have the same qualities. And I will never have 8 grand for a bike unless I win the lottery so it will be my only route. But I would only do so if I could find something that had not very many miles put on it. The older it is the more likely someone might have stacked it and although that might not have happened it isn't something I want in the back of my mind while on a fast descent. I want to feel confidence in the bike I am on. I'd have less on an older bike with an unknown history. I got very lucky with what I've got now.
Also I'd prefer to source f + f and build up with up to date components, but you can still get a top end bike for the same price as that PX. So it depends what you find out there and how long you are prepared to keep looking, but there will be some hardly ridden bargains out there that despite being a few years old, will ride like a current top end bike IMO.