China's economy might be No 1 in 2030

One possible crunch time for China could be around 2015 and 2016 when a lot of local government debt - where China's overall debt has become centered - has to be repaid or rescheduled.

"People have kept reminding me since I wrote the book that the investment bust hasn't happened yet but there is always a risk of it and that would slow growth."

Magnus, senior independent economic adviser at UBS in London, believes it is possible China could become the world's biggest economy by 2018 and twice as big by 2030, but they are more like "spreadsheet" calculations.

"There is a danger of it being just a mathematical exercise. You could have a scenario where China grew by 8 percent one year and then by 4 percent in four successive years or smoothly by 5 percent in five successive years. You would get the same outcome but it would be a very different type of growth since the first type would involve a significant slowdown."

The quality of China's growth is something that concerns Gary Liu, executive director of CEIBS Lujiazui Institute of International Finance in Shanghai.

"The problem is not about how big the Chinese economy can become but the quality of that growth.

"I believe that quality is bad in China and it has been at the cost of people's health. You can feel how bad the air and water quality are. I am afraid in the coming years we will see more people get serious diseases - even cancer - in China. This is not a small issue."

Liu believes Hu's prediction of China becoming double the size of the US is off the scale, and he doesn't expect China to even get to the same level until 2033.