China's decision to open its stock market to foreign investors will impact the US Economy, but only marginally. The Chinese government is well known to have very stringent controls throughout the country, so in order for an impact to be dramatic, those controls would have to be lessened- a task that the Chinese government has never seemed willing to partake in. There will likely be some initial investments made by US investors in China, but it will be a minimal impact.
The opening of the Chinese market provides American investors with a new place to buy stocks, meaning we may be witnessing a way to revive the American financial industry. Foreign trade allows us Americans to have a new place to focus our energies. If American companies can make money in China's stock market, and we Americans reap the benefits, I'm all for it!
It seems that every year, the integration of formerly national economies into a largely global economic system takes one more step, and the opening of the Chinese stock market to foreign investors is another huge development in this progression. With the new option to put their money in an often hot Chinese economy, large scale investors may now pass up shakier investments in the US for these other options, which obviously will affect the US economy.
No, I do not believe that China opening its stock market to foreign investors will impact the US economy. Investors will always find a way to invest in forign markets, and I do not believe this will have any impact on the US economy as the money would have been invested somewhere, why not have it be in China.