merchants would not convert btc>fiat using public exchanges anyway. + we all seem to look closely to these type of exchanges and their prices but it aint were most of bitcoins are (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.