Perhaps someday we'll discover a coffee tolerance gene.

The gene is called CYP1A2 with two variants. One variant is a rapid caffeine metabolizer. The other is a slow caffeine metabolizer. https://www.geneticlifehacks.com/liver-detox-genes-cyp1a2/

I have the rapid variant but could not sleep well on 6 cups.

That is interesting information. I note that the same enzyme breaks down melatonin. If a variant that is particularly efficient at caffeine is also particularly efficient at breaking down melatonin, the symptom would be difficulty getting to sleep independent of amount of caffeine, which is totally fixed by taking melatonin tablets.

Slow metaboliser here. If I drink a coffee at 11am my heart rate rises and doesn't return to normal until the following morning.

There are other genes for it too, like one that affects the degree to which coffee causes anxiety. And plenty of others that we haven't found, presumably.

I love love love love coffee. It took me years to realize that my high blood pressure was controlled almost immediately by cutting out a high-test canned “Draft Latte” drink. I can drink a single 8oz cup every few days. Decaf is fine.