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1kg isn't noticeable as long the bike feel light and easy to climb.
Especially if it's something like the CAAD12.
As @Shroom quite rightly said, it's very likely to be just the placebo effect that you end up believing that it's the 1kg that held you back, not your fitness levels, and maybe the bacon baps you had for breakfast.
edscoble-
Depends how lean you are and what you're using it for I guess. I don't think I could loose much more weight but if I'm racing on a hilly course I'd defo want my SS HM over my Canyon Ultimate AL SLX. Reckon there's about 1 kg between them and both are kitted out fairly well. SS is stiffer but without doubt there is a certain placebo effect too.
My CAAD 10 ownership lasted all of 5 mins riding (before it was written off) so can't really comment on the CAAD 12's predecessor but the Supersix climbs beautifully. Mine is 6.3kg for a 54cm v the 56cm CAAD 12 Ultegra Disc 3 (8.34kg) or the non disc (7.5kg).
Looking at that you'd really notice a 2kg difference! (couldn't be arsed to search for 54cm weights).
Currid
If you're tackling alpine descents, that means you're also going up them. In which case 1kg is very noticeable. As nice as the CAAD 12 may be, I also wouldn't want to drop £2k on an alu bike.
I'd hunt around for an older Supersix model on discount. I picked up the 2013 HM for around that last year, and tire clearance aside (I'd like more!) I'm very happy with it.