Boss, quick question: What are the pros and cons of rubber spring cushions for lowered vehicles?
There are several types and brands of rubber spring cushions available, so it's difficult to give you an answer based on the short description you've given me. However, if you're looking to improve the ride of a lowered car, then you're better off getting lowering springs that have similar spring rates as your stock springs.
The spring cushions you're referring to may absorb some noise, vibration, and harshness (NVH), but it won't soften the ride of stiff springs. Moreover, cars that are lowered by just cutting off one or two coils from the stock springs tend to be stiffer. And more often than not, they don't sit properly on the spring seats.
What are the pros and cons of aftermarket spring cushions? They can surely eliminate some NVH, but if you choose the kind that sits in between the coils you're preventing your springs from working as well as they should, and this may even lead to a harsher ride. I would prefer spring cushions that sit on the spring seat and act as an additional damper between the spring and spring seat. This could help in supplementing isolation and diminishing some NVH. Other than a lightened wallet I don't see any cons to this mod.