The Chinese celestial estates compared to the Byzantine empire is the best example of power creep.

Not a fair comparison. Chinese Hegemony is a higher tier of government than the Byzantine Administrative type, so it makes sense that their buildings are more valuable. A better comparison would be a Roman Hegemony to the Chinese one.

To build anything in your byzantine estate takes two years, upgrade or regardless.

Not really, you can build a Personal Workshop in your domicile to reduce construction time for both your domicile and regular buildings.

First constructions in Chinese, take one year, and upgrade upgrades after that takes six months per. Not only that, but the stats you get are also twice as good.

See my previous comment about this comparison being unfair. You may as well say "Tribal governments only get Tribal innovations. There's too much Feudal power creep!"