I have to agree with pretty much everything that others have written, especially fearfactoryüberalles: frame weight is important but so are other things, especially how the bike fits you. Ultimately, I think, having a bike that you really enjoy riding is a very good thing: it can help add years to your life and improve your quality of life as you age. The weight is just one contributing factor. Having said that, my Ridgeback is nothing exotic but the frame weight is only about 1.5kg: I think Univega used the same Tange 0.9, 0.6, 0.9 'Racelight' tubing as well on some of their bikes. Both tend to sell for very reasonable prices.
However, I think that one of the most remarkable builds on Retrobike began with a humble HardRock: they didn't have the lightest of frames, but see what this guy did:
http://www.retrobike.co.uk/forum/viewto ... mp;t=24317. And someone here is winning races on an old HardRock:
http://www.retrobike.co.uk/forum/viewto ... p;t=387517. I'm not saying that you should rush out and buy one of Specialized's lowest - just bear in mind that there is no real 'Ultimate' retrobike: it's different bikes for different folks. You might want to check this useful article, too, if you haven't already:
http://www.retrobike.co.uk/retro-bike-b ... #more-3093.
In the end, you've got to get out and ride and then you'll find what works best for you, whether that's exotic or humdrum.