The HHKB type S has a 3mm travel vs. 4mm reg. model, the sound reduction is on the upstroke. I don't think most would feel a 1mm difference IMO or better feel, it just silencing key return.
I just tested the key travel on my 87U silent and HHKB non-silent boards.
My four month old Realforce 87U Silent and three week old HHKB Pro 2 non-silent.
This is the depth gage used. Measurement unit is millimeters.
The test method was simple. The base of the depth gage sits on the adjacent keys. The plunger is placed in the center of the key being tested and depressed until the key bottoms out.
First test was done on the 87U Silent. You can see it reads exactly 4mm.
Then I tested the HHKB non-silent. You can see it's also exactly 4mm.
Test Results:The key travel is exactly the same for silent and non-silent Topre switches. There is no difference in key travel between the two. Which means, whatever Topre did to silence the switches, it's not simply a factory mod like we would do. Also note that the silent switches have a purple housing. Non-silent key housings are black. Read here for more info on Topre key types and colors:
http://deskthority.net/wiki/Topre_switchGiven the different key switch housing (differentiated by color), and my key travel testing, I'm going to go out on a limb here and conclude that the engineering is different between the two switch types. If they do use some sort of dampening material, they've also changed the switch housing to accommodate it, resulting in identical key travel as their standard switch.