I have wondered this as well. According to
http://www.numbersleuth.org/worlds-gold/AboveGround.png
Only 12% of gold is used in industry. Unless I am mistaken, I think this means that gold's price would be ~12% of its current price were it to be used only for industry (massively simplifying of course).
The chart is more enlightening than this number. Jewelry accounts for more than half of the gold above ground. I have changed my understanding of the problem. The question should be: what would happen if gold were used for everything but "investment". That leaves us with 16% surplus of supply over what we have today. The market cap of gold is 5 trillion dollars. If 16% of that worth went into bitcoin it would have a market cap of 900 billion dollars!
sdp