It’s tough to really quantify since I’m not doing any specific testing to evaluate my performance with/without. I know that caffeine is highly susceptible to tolerance buildup, so I do occasionally skip the added caffeine. My usual does is 160mg of caffeine to 320mg of L-Theanine, taken in a combination capsule form from www.powdercity.com (also sold under a brand name “smart caffeine” but it’s the same stuff for a bit more money). The effects of caffeine + theanine are so smooth that it’s easy to overlook them. Mostly at this dosage level I just find I feel slightly more “up” for the day, but nothing super extraordinary… just a pleasant boost.
Once I increased the dosage to 4x capsules (320 mg caffeine and 640mg of theanine) and found that was a bit too much. My eyes kinda bulged most of the day and I felt uncomfortably amped… though not as wired as I would be from say a large Starbucks Frappacino. I now stay under 3 capsules as my upper limit, but most days stick with 2 in the morning, and occasionally 2 more in the afternoon if I need to stay up and work late.