My sweet spot is around ~7 hours. Less than 6.5 and more than 8 makes me feel the same, but if I manage to sleep around 7, it's completely a different story.
I was drinking 2L/day at my peak, while I was sleeping 6 hours a day (Ph.D. corrections). It gives you awakeness, but the not the concentration of a well rested mind.
Nothing replaces sleep, since your brain flushes its toxins through your spine to be expelled from your body.
Sleep is underrated and deprivation of it kills you slowly.
I work way better with half a mug of coffee (a Starbucks tall) and proper sleep than two Ventis and not enough sleep.