There are some posts on GH and/or DT about transferring NT sliders into other Topre-switch keyboards. Unless you are quite keen on other form factors or being able to put Cherry mx keycaps on other Topre-switch keyboards, you might want to keep boards such as the HHKB Pro 2 or FC660C as they are.
Regarding improvements for the NT (as well as other Topre-switch keyboards), I've produced Silencing Rings and installed them in my NT, HHKB Pro 2, and RF87UB55. I am quite pleased with the results. You can find the rings by going to eBay and entering "Silencing Rings" in the search box.
There are also alternative silencing treatments for Topre switches, such as ironed soft landing pads and dental bands.