If you have a company you can ship it to, you can still buy pure caffeine powder from bulk supplement retailers. (No one will sell to individuals anymore because of the safety risk.)
That makes it a lot easier to meter dosage for an experiment. You can also try to get consistent caffeine by using a consistent, fresh bean/roast, high quality burr grinder, and a brix meter to check extraction %.
I have little doubt that pure caffeine would impact me. My question is on the rush from coffee. To me, it is just a tasty drink. I don't feel more awake after it. At all.
That makes it a lot easier to meter dosage for an experiment. You can also try to get consistent caffeine by using a consistent, fresh bean/roast, high quality burr grinder, and a brix meter to check extraction %.