What is an English word to mean “something that makes already strong one much stronger”?

Solution 1:

go from strength to strength meaning progress from one success to another higher level of success, or continuing to grow stronger.

http://www.phrases.org.uk/meanings/144200.html

eg. With the ongoing military modernization, China is now seen as moving from strength to strength.

Solution 2:

I think one of the most common phrases you would hear to describe this in American English is the rich getting richer. You can apply it to your example and it has wide usage.

Solution 3:

Since the question says phrase, from Austin Powers: "sharks with frickin' laser beams attached to their heads". Something already strong and feared topped off with something stronger.

Solution 4:

There is a Chinese idiom literally translated as adding wings to a tiger (如虎添翼), which also means exactly the same thing as 鬼に金棒.

This may be a good phrase to use in English because people unfamiliar with the idiom are still able to guess exactly what it means, whereas the literal translation of 鬼に金棒 may not be so obvious in its meaning. For example, see its use in this article: Like Adding Wings to the Tiger: Chinese Information War Theory and Practice

Solution 5:

One relatively recent slang usage in this general area (which admittedly isn't exclusively applied to things that are already strong) is...

a bear / elephant / bull / etc. on steroids. (on steroids = in a much more powerful or extreme form)

For OP's exact context, a few people have actually written China could be like Japan on steroids.