I've been roading now for three years and just went from a @20lb bike to a @15 lb'er. What I;ve noticed is not so much the weight differential, but the handling of the lighter bike in two specific instances.
1)on downhills the lighter bike goes as if its on rails.
2)when accelerating, the newer bike feels more explosive, responsive.
This may not be due to the difference in weight so much but in the design aspects which are engineered into a lighter bike.
hmmmm...could also be the difference between a Ti and a carbon frame as well.....

Best advice ever : HTFU and get out and ride. Only way to get faster and to get a real feel for what your bike is capable of.