So,
I have completed the silence mod on my FC660C. It is epic. But I have some advice for others: get the soft-landing pads from EK. Don't screw around with anything else. I used adhesive rubber pads from Ace that I punched and trimmed to size and it was a royal pain in the ass. Took way longer than necessary. Also, these were only about 1mm thick, but they are very nearly too thick. In fact, for some keys that were particularly sensitive, I had to sand down the rubber pads to avoid false keypresses. I also had to tighten PCB/plate screws a minimal amount to give the domes as much free play as possible. Even with this, many keys are on hair trigger status due to the extra dome compression from the pads I inserted.
I believe the soft landing pads are thinner, and either way, they'll save you a huge headache doing it another way.
ANYWAY. Here are the vids. A note in advance-- the FC660C has a voice for radio. IMO, the stock configuration sounds much better on a recording than in real life, including on my 'before' video below. This is true of nearly all Youtube videos of the FC660C. Most recording devices seem to overemphasize the thock and underemphasize the click (upstroke) which is really not accurate at all. Even when trying to avoid false bass detection, videos make the FC660C sound like it has this great rolling, resonant low tone. NOT TRUE IMO. In reality, the upstroke click (yuck) really overpowers the downstroke thock; hence, the silence mod.
The videos are a fairly close approximation, with the caveat I mentioned above. FWIW, In real life, the silence mod sounds way way way better than stock.
Also, not that I did not silence the backspace key.
Vid 1: FC660C before modding 
Vid 2: FC660C after silence mod 
Great job, and thank you for posting the videos.
The soft landing pads are not thinner, as they are approximately 1mm thick, but they are probably made of a softer material than the pads that you have used. So the rubber dome and the spring of the Topre switches are strong enough to compress them just a little, and eventually everything falls into place.
My mod of the FC660C was one of the first I have ever done on Topres. I think I may have done the mod on my Realforce 88UW before, but I'm not even sure.
Anyway, now that I have more experience, here is what I would recommend:
1. Use EliteKeyboards' soft landing pads, because they do the job and I don't see any other easy way to agree on a useable and well-defined material.
http://www.elitekeyboards.com/products.php?sub=access,slpads&pid=sl120_csNOTE: O-rings are not suitable. They grey landing pads are not suitable either.2. You need to slim down the landing pads using a clothing iron. Yep. You do that by putting the landing pads between two sheets of paper (preferably non-printed) and press on them with a hot iron until they are slimmed down to
0.5mm or even 0.3mm. You will need to make a number of trials in order to find the right temperature, the right amount of pressure to apply and how long to press on them. I slimmed them down 4 at a time once I had found the right way to do it.
The slimmed down landing pads work really well. The reduction in travel is barely noticeable. I have been forced to start using this technique when I tried to mod my Realforce 87U. It has lighter keys than the FC660C (some of them are only 30g), so using out-if-the-box landing pads did not work. I had a small number of keys that stayed actuated.
Slimmed down landing pads work on 30g Topre switches, and they work even better with the 45g or 55g.
In 30g switches, they change the feel a little bit. On 45g and 55g switches, they make almost no difference in feel. But they do make a huge difference in sound: the famous "Thock" is still there, but the plasticky "click" on the upstroke is almost gone.
I can't resist to post once again the video demonstrating the difference between a stock Realforce 87U and one silenced using slimmed down landing pads:
http://www.chesstiger.com/images/keyboards/RF87_silenced_before_after.mp4Finally, one thing I have noticed in your videos is that your spacebar seems a little noisy. I think you need to put high viscosity lubricant on the stabilizer wire, at every point where it touches the plastic, that will help. You may also need to put landing pads (or something equivalent) under the spacebar, on the stabilizers plungers.
