I ordered some balls a while ago, but they haven't arrived yet. I will be posting my experience with them in the trampoline mod thread.
1. They are not silica gel, but rather silicone rubber, so they will be soft like the oring pieces used in the trampoline mod. However, they are a different size and shape, so will feel a little different. They will deform to fill the space under the slider "pole" more quickly, so will probably be a little softer initially, but resistance will increase more suddenly as they fill the available space (when a there is no space available to deform into, there is no possibility of more movement).
2. Soft, so no sound added.
3. Two balls will raise the "bottom out" point on tactile and linear switches by around 2.5mm (2mm diameter X2 - 0.8mm space below slider pin - squish amount = 4mm - 0.8mm - about 0.75mm overall = 2.45mm), which is above the actuation point, so I don't suggest using more than one. I estimated the squish amount, though, so it could be closer to the actuation point, perhaps even right on it, but it probably wouldn't feel nice to have to push so hard to get the switch to actuate. So I don't recommend using more than one. On clicky switches the slider pin is longer, so they will raise the bottom out point more.
4. They will not melt and deformation will be minimal as silicone rubber is quite durable, but they may well conform over time more to the shape of the hole and provide less rebound / damping.
5. I will be trying this mod when my balls arrive, so I don't consider it retarded or stupid. I suspect I will prefer the hourglass shaped "trampolines" (cut from 2mm silicone rubber sheet with a leather hole punch) which I am using on my KBT Pure with Browns, though, due to the limited squish area available to the ball shape.
6. Check the trampoline mod thread for my results once they arrive.
They are cheap, so worth trying IMHO. Try on a single loose switch first if you have one to get a basic impression of the feeling. Biggest downside of the mod is you have to open up the switches. Biggest plus side (compared to orings) is it's independent of the keycaps and gives a consistent feel across the whole board.