The bike's weight, much like the rider's weight does make a difference.
I have no idea how you would compute a "percentage" of factor an improvemnet for a given course. However, laws of physics support two major advantages to current riders.
1
Small lightweight riders, 130-140 lbs enjoy a larger advantage in weight savings than big 170lb riders. So a lighter bike makes an even bigger difference for them.
2
Lighter wheels reduce the amount of energy expended across a given course, the smaller the rider, the hillier the course the more benefit.
My own suspicion is that STI-style levers and better gear selection offer time savings in almost any situation. How much? Who knows?