I have several RF 87U variable silent boards as well as an HHKB Pro 2 standard. I liked the idea of the HHKB Type-S, but didn't want to spend the money for it. And I've never considered doing a silencing mod to my standard HHKB simply because I didn't want to change the key travel.
The Realforce silent actually uses a different switch housing, so that you still have 4mm of key travel with the silencing. I always assumed the HHKB did the same. However, according to PFU's specs, the key travel on the Type-S is reduced to 3.8mm. I'm not saying that reduced travel is good or bad. But my point is, if you want an HHKB Type-S, I see no reason not to just do a mod, provided the the foam rings are only .2mm thick.
Here's the link to PFU's HHKB Type-S specs...
http://www.pfu.fujitsu.com/hhkeyboard/hhkbpro2_types/feature.htmlFrom what I can see, I don't think it would be any different than the factory Type-S. Am I wrong?