Problem with this is that it is totally impossible to collect ALL traffic, even for the NSA. There has to be some kind of filtering between where they tap into the network and where they store the information they collect. Not to mention that they are not Santa Claus, they don't have 100% coverage of all internet traffic.
With that in mind, tell me why the NSA would be interested in watching Mt Gox? Because that's what you are really saying, that the NSA was specifically targeting Mt Gox and/or BitCoin transactions, which may or may not show up where their probes can see it. I don't think the NSA cares, and was/is not in a position to monitor all BitCoin transactions.