Nominal GDP is based on three factors.
1. Real economic growth
2. Inflation rate
3. Currency appreciation
China is currently using the archaic SNA 1993 accounting method. Everybody else is using the SNA 2008. There is a 200-page study by an American think-tank that shows China's SNA 2008 nominal GDP would be $1 trillion higher than the current figure.
China had announced its currency will be free-floating by the year 2020. In four more years, China can no longer control its exchange rate. China has a merchandise trade surplus of $600 billion in 2015. With oil dropping to $30 per barrel, China's trade surplus this year should be around $750 billion. By 2020, the pressure would be enormous for a massive appreciation in China's currency due to the accumulated trade surpluses.
In conclusion, China's nominal GDP could overtake the US as early as 2020 (due to a market-based currency valuation and adoption of SNA 2008 accounting method). At the latest, I would say about 2025.
7 years left for Xi Jinping.
Economic performance is based on the metric that you choose.
In terms of absolute economic growth, Xi Jinping is presiding over the largest economic growth in China's history at $1 trillion nominal GDP growth per year.
In percentage terms, Xi Jinping is presiding over the smallest percentage growth due to China's existing massive nominal GDP economic base.
It depends on your frame of reference. Do you care about absolute economic growth? Or do you care about the statistical percentage growth? Xi Jinping has to grow off of a much larger GDP base ($11.4 trillion). Thus, Xi Jinping will be stuck with a lower percentage figure. This large-base statistical effect applies to all future Chinese presidents.