You'd be hard-pressed to find a faster fan, those puppies are Deltas. It's a custom one though, there's two deltas side-by-side in a single oval housing, you probably won't find a drop-in fan to replace it. For an easy solution, I would take off the heatsinks and apply some decent paste, the fans are thermally throttled (you haven't heard half their volume yet). So by keeping it cooler you'll make the fans work less, and thus quieter. If you really want to replace it, I think you'll be hard-pressed to find an 80 (?)mm fan that's quiet and still good performance. Anything with decent airflow in an 80mm size is a server fan, and thus loud by design.
Apparently it's 60 mm's, disregard what I said about the size because that was from recollection.