So after browsing this forum for a few weeks, my interest in topre switches have grown exponentially and now I want to give them a go. However, with the high price point that these topre keyboards (I have been mainly looking at the FC660C and HHKB) have, I have a few questions that hopefully you can answer before I start spending the cash.
1. I have seen people describe the feel of a topre as having a tactile bump when it comes back up. Where exactly does this tactile bump come from? Is it from when the plunger comes back into contact with the housing? Or is it from the resistance of the rubber dome similar to the traditional membrane keyboards? I am having a hard time imagining what the bump could feel like if the main moving parts are the rubber domes and plunger.
2. How similar are topres (55g or 45g) to lubed stock mx clears? I know it's hard/useless to compare cherry mx switches to topres, but these are my only point of reference (and traditional membrane keyboards).
3. What makes the Leopold FC660C often seen as inferior to the HHKB (assuming that popularity = quality)? Ignoring the aesthetics and form factor, are the quality of the keyboards vastly different? Because I plan to install novatouch stems and 55g rubber domes into either the Leopold FC660c or HHKB P2.
4. Does the Leopold FC660c have random stem placement in its spacebars in the same way the FC660m has for the cherry version? Keyboard customisability is important to me.
And finally just to clarify, do the topre keyboards use different spacings for the stems i.e. there's no point installing the novatouch sliders for the spacebars as no cherry spacebar will ever fit.
1. Here's a good illustration of the "bump" in action

2. Topre is going to feel closer to a traditional membrane board than anything MX. In fact, think of Topre as a very high quality rubber dome - if rubber dome is like a Toyota Camry, then Topre is a Lexus LS. If your only MX frame of reference is Clear, then you'll probably want to stick to 55g. I personally am not a fan of heavy switches of any kind, so I'm sticking to 45g. They are definitely soft but not at all what I would consider "mushy."
3. While I've not owned either the FC660 or HHKB, they are both regarded as having excellent build quality. The difference is in case versus plate mounting, as has been pointed out already. Discounting the fact that I own two Realforce keyboards (plate mounted) and going purely by sound, plate mounted is more appealing to my ears. This is the result of listening to many typing videos of the HHKB (Type S included), FC660, and Realforce. It all comes down to personal preference.
4. Others will be able to provide better guidance here, though I have to say what you're proposing is going to be extremely costly as you'll need to buy three keyboards. The FC660C and HHKB are running roughly the same price ($250 give or take); the Novatouch is $160-200; and you'll need a 55g Realforce, which is another $215. Naturally, it's your money, but it might be better to go one step at a time. Start with the 55g Realforce to see if you like the weight of the domes. If that's cool, grab either the FC660 or HHKB to decide on a layout, then the Novatouch to do the mod. If you end up not liking the 55g, it's going to be very easy to sell it here and get back most of your money. Then you'd have to decide if you want to try a lighter Topre switch or stick with MX.
Good luck with whatever you decide.