Empire is not single country or state. Just like British empire, Chinese empire should be considered the same way, which includes its homeland states and its colonies.
Historically and culturally, north and east coastal region of China is the homeland states (similarly like Scotland and England). South and west Chinese provinces are really Chinese colonies.
The homeland states are often in political and economical control. Homeland states are also wealthy in term of GPD per capita. On average, with its colonial states in west, China is low in term GPD per capita. If organized the same way as British empire, Chinese homeland states like ShangHai regions should have GPD per capita in the wealthy countries category.
If China keeps together with current demography, GPD per capita will never reach top of world. If each province have its own economy independence and currency, homeland provinces of Chin might have been in the league of developed countries.