TDF is pretty much the cutting edge of measuring performance and how things have changed over the years.
TDF stats:
http://www.bikeraceinfo.com/tdf/tdfstats.html
Just compare the stats from 1980's and 2000's... Given that bikes have changed monumentally in the areas of weight, aerodynamics, and materials for the sake of speed, performance and efficiency, the change is not really that monumental. Getting the bike to drop almost half the weight is the biggest factor in about 1.5 mph increase on an average, and that's probably all gained with climbing.