I used Visa in Shanghai in Feburary. The rate was 1CAD vs. 6.01RMB. There is NO additional charge what so ever. I also exchanged some CAD cash in Bank of China. The rate was 6.09. At the same time, the rate in Accurate here was 5.5RMB vs 1CAD.
Remember one thing, it's always cheaper to change CAD to RMB in China than in Ottawa.
You would have got more money if you had exchanged it in Shanghai than using the VISA.