I strongly believe that if some well known investors come in and buy billion dollars worth of bitcoin it could be world breaking news and the price of bitcoin would soar high to $10000, just my opinion.
I think it will depend on how he will be usinnthose coins, that is a lot of coin and it will be affecting greatly the price of bitcoin, but you must also take note that the distribution of bitcoin to users are making the price pump like the signature campaign and bounties. And even if a known investor bought that amount of bitcoin, yes it will be having a pump but not that huge effect.