Well at a multiday ride last year the following happened. A guy I know had a brand new carbon fiber bike that was very light. So Diane took the sucker apart and filled the tubes with rice about 2 lbs of it. The guy did not notice anything different until miles down the road he happended to notice that one of the bar end plugs was a bit loose. He pulled it off and out came rice! With Diane's help later, he managed to get it all out.

I suspect that very few riders can tell if a bike is a lb lighter or not.

I did pull off my frame pump, tool pack and water bottles for a short race once. I noticed that the bike did definitely feel lighter. But all that stuff probably weighed over 5 lbs.