USB drive speed depends ALOT on the controller used and the device used that connects to the usb port.
How many MB/sec do you get on your xbox 360?
USB 3 is not easy. Nintendo would have to make an "official" Hdd, or else there will be alot of confusion. ALOT of lazy companies sell their USB2 drive with a sticker on the package that says "USB 3 Compatible!" and the consumer in the end wonders why its so slow. Belive me, not every customer knows that USB3 connectors are blue and have a second set of connectors on the inside.
Another problem is that some USB3 drives still are pretty slow and only slightly faster than USB3. "Official USB3 complaint" sticker is a nice selling feature aswell

I have a WD MYbook USB3 drive wich can hit 100mb/sec read and write, but it cost quite some money. (Ok its a 2 TB drive...)
Making an official "512GB Externel Nintendo HDD" wich has some speed behind it would be a good way to avoid it. If they just allow HDD install and let the costumer, that is not as "good informed" as we, the nerds/geeks, buy and connect any drive, then there will be confusion. Heck in very few instances the drive may not even work (Even though very unlikely, but hardware incompatibility CAN happen. I had this once with an external drive... Worked fine on my dads pc. Wouldn't work on mine).
Well lets sip some coffe and wait for E3. We should know alot more after June 5th!