If you wants China surpassing the West, China growth must be more because the West = US+EU+Australia+NZ (even Japan, South Korea also counts). China has more population than the entire block and China should have a larger economy than the entire block, not just US or EU. 5.5% this year is slow. If this rate continue, it will take another 2-3 decades for China to surpass the entire block.
While the West = US+EU+Australia+NZ+Japan+South Korea is inded larger than China by quite a bit.
But I don't think it's really a case of 1 + 1 > 2 nor even 1 + 1 = 2, yes I think the whole west, as the countries gets 'added together', there's a 1 + 1 is smaller than 2 effect.
China is
ONE large rather cohesive country.
While the various states, local governments etc. in China does have some autonomy, along with, ofc, power struggles internally in China. At the end of the day the people inside all recognize themselves belonging to the country China (not to mention the central government in China having strong control over lots of SOEs, including very big and critical ones that have strangleholds over many important sectors).
While 'the West' on the other hand, we have a large collection of many countries, which does center around the US, but the US ability to command and control those countries is weaker and worse than the control the Central government in China has over local chinese governments.
Not to mention control over companies, economy etc. (and 'the West' would also have worse problems when it comes to internal power struggles, both internally in each country but also between the countries, especially as the US is starting to leech off the other countries to try and strengthen itself)
Basically, one's a single big say 1m diameter rock while the other 'rock' might be 1.5m diameter, but it's actually made of multiple pieces, all of different sizes, badly bound together to form that 1.5m diameter rock (and the biggest core rock is at the same time trying to cut off pieces of the smaller rocks, to somehow absorb into itself).