Yeah... true!!! But what we do know, is how we feel from our own personal experiences, dabbling.
So with that, for me, i can definitely say i felt an increase in performance on endurance sessions. Percentages are however not straight forward to calculate, without the right tests being done. But, i would say i easily felt a 20% improvement, over endurance sessions where i had not taken in any caffeine beforehand.
Male, 39, 5'10, 75kg (Started rowing on June 9th 2016)
PB's (2k)7:23.2 (5k)19:48 (6k)23:58.8 (30:00)7518
2014 model E
Rest in peace Christina Grimmie 1994-2016. Heaven gained another angel