If you are completely crazy about fast lenses that you should take a look on this. The Photographer Aldo Navoni used the De Oude Delft 50mm f/0.75 (!) on the Sony NEX-5. It might be worth t take a look on his Flickr page (Click here). I don’t know if there is any faster lens on earth you can use on a NEX!!!
You can get the lens on eBay (Click here). But damn expensive!!!!