(there is around 2 to 3 millions BTC out of 13million total on public exchanges).
I very much doubt it's even close to your figure. Post MtGox, probably less than a million BTC, and I think even 500k are on the exchanges.
It may be; seems hard to tell. (I would think that day traders (should we call them "minute traders" in the case of bitcoin?) will want to keep all their speculation budget in the exchange, in spite of the risk, since they may need it without prior warning and their revenue is proportional to the amount traded.)
A more important number is how much bitcoin is held by speculators who operate on the scale of days or weeks, rather than minutes. Those traders probably keep their coins and money off-exchange, but will move them in if the price changes significantly. Thus the walls at 800$ or 400$ may be a lot taller than what shows in the order books.