Something else just occurred to me -- first, both the Warring States and Hellenistic periods have reputations for being periods of lots of technological and scientific advances; it is frequently speculated that prolonging either of these periods could have led to even more progress in this (some going so far as to claim an earlier scientific revolution might have resulted).
And here we have a TL that not only does this, but potentially makes Northern China part of an expansive Central Asian empire some 1400 years earlier than OTL, allowing these hotbeds of philosophy, experimentation, and invention to potentially exchange information and ideas at levels U reached OTL until centuries later. We could see things like finery forge iron arriving in the west over a millenium earlier than OTL; that one invention alone could mean, for example, more effective farming techniques, further boosting these developments.
Another thing to keep in mind is that (ridiculous levels of tech advancement aside) TTL is likely to see the same global climate changes as OTL; meaning, shortly before our PoDs, we're still seeing the start of what OTL calls the Roman Warm Period. This will still last about six centuries or so, meaning wherever Europe and Asia are as of the 3rd to 5th centuries CE is still going to be entering something of a dark age.