A student from the local college asked us in the running club to test-out the effects of caffeine.
We ran a measured course one week, were timed, heart-rate measured at the end, said 1-to-10 how hard we'd gone.
The following week, half got a caffeine pill equivalent to about 4 espressos, half got a placebo and we repeated it.
Result : everyone went faster the following week, we all said we'd gone faster but found it easier...
But yes, caffeine is an experimentally-proven ergogenic aid and is on the banned list, but only if you have a lot of it in you : a couple of coffees before the start of a stage wouldn't put you over the limit.