Yes, Sanwa buttons are much quieter than Happ buttons. They lack the microswitch "click" common to Happ buttons. As you mentioned they are standard in the new Tekken5 units, as well as being pretty common in the Japanese "candy" cabs.
You can get screw in and snap in. As mentioned, both are designed for sheet metal (or heavy duty plastic) cabinets of only a few millimeters thick, as that is the standard material used in Japanese cabinets, as opposed to the standard 3/4" MDF or ply on older US and European cabinets.
Himura Amusements sells these buttons, and you can pictures of the buttons on their site:
http://www.himuragames.com/