Join Date: Jan 2008
Location: Beaverton, OR
Bikes: 2013 Kona Jake, 2015 Kona Jake the Snake, 2008 Kona Major Jake, 2013 Kona Jake the Snake, 2006 Kona Kula, 2012 Ridley Excalibur, 2008 Surly Long Haul Trucker, 2001 LeMond Buenos Aires, 1984 Pinarello Gran Turismo, 1982 Trek 614, 1979 Austro-Daimler
Typically when your bike is heavy it's because your parts are heavy. A really heavy frame typically weighs no more than 4.5 pounds. All but the very lightest frames weigh over 2.5 pounds. A steel fork might weigh as much as 2 pounds, and a full carbon fork will generally be just under 1 pound. So going from really heavy to really light in the frameset might save you 3 pounds. That's nothing to sneeze at, but you have to decide for yourself if it's worthwhile. Also, I think you've already got a carbon fork, and I'd be surprised if that frame weighs over 4 pounds. You probably would be saving less than 2 pounds.
That Colnago frameset won't work with disc brakes, so that's probably a consideration for you. Most carbon framesets these days will have some kind of oversized bottom bracket and head tube, so those are additional parts you'd need.
Making a bike lighter is always a very expensive project, and it's not done until you've replaced everything.