I realize this is an old thread, but it ranks high on searches.
Usually, I do not touch Kava on workout days until after my workouts. Occasionally, I will drink a cup very early in the day--many, many hours before my workout. On two occasions, I have had many cups before my workout, and the hours I left between drinking and lifting weights were not enough. On both occasions, I was very concerned about lethargy, form, weakness, and nausea. On both occasions, I was pleasantly surprised.
I did feel lethargic both times. That said, my form was fine, and I felt much more in command of the weight. I did not feel the normal level of challenge, and when I do not feel challenged at my target weight and rep range, I sometimes extend the last set to see how much I can do before it becomes challenging. Today, all of my lifts felt much less challenging, but I only attempted to extend the bench press as the others are more complex and more likely to injure me. On my last set, I increased my reps by 60% without a breakdown in form and without going to failure. Rep 8 out of 5 was challenging enough that I knew it would be foolish to attempt a 9th, so I stopped there.
There were two downsides. I definitely was lethargic, though this did not impact my lifts. The other was some forgetfulness. I would focus so heavily on the lift that I would sometimes forget which rep I was on. For example, on the 2nd to last set of bench presses, I lost count on my 5th rep. To be sure, I did another rep, and my spotter informed me that it had been an extra. So, in total, that was an extra 4 reps at my target weight, or a total increased volume of 27% without adding another set.
I would never recommend doing this intentionally; however, I think it is a fascinating anecdote. I will continue aiming to enjoy my Kava after my workouts.