A Cannondale “2.8” frame is literally that, 2.8 lbs, so there’s your weight saving right there.
Seriously, though, what is this steel bike you’re looking to upgrade? Depending on frame size and material, there may only be so much weight you can save with components alone.
I have found, though, that going from a vintage style 33/36-h box-section wheel, to a modern semi-aero (20-30mm) 20/24-h wheel and a good-rolling tire like a GPx or UltraSport makes the bike feel ‘lively’ and more responsive to acceleration and handling. It may not save any real weight, but it makes a difference you can feel.