I think T84 is the best I can find. (If T83 is possible somehow, it would have to be something very extreme!)
I tried doing AH since Tokyo and Osaka have cows. The cities develop more since they have much more food and production, but the initial delay (since you have to research Agriculture -> AH) means CS comes a few turns later. Without much optimization, I got T90, with Kyoto and Osaka at population 8 and 7, with Buddhist and Confucian temples in both.
I also tried doing Agriculture -> Pottery -> Mining, but this was about half a turn slower than Agriculture -> Mining -> Pottery. There just isn't a way to develop as quickly if you delay Mining the silver. For example, you can delay your settler until size 4, but then, even though you can get a GS a turn earlier (1440 BC in the logs), and you can bulb math a turn earlier, your second city is settled about two turns later, and the second city isn't able to develop quite as quickly, so the CS tech time becomes about a turn and a half slower.