I'm sure you could turn up some info on a search, but this is worth discussing.
OEM "cabin comfort" criteria for any vehicle is a mix of many factors. Reality is, the Caprice was intended to have a fairly isolated passenger compartment, with as little intrusion of noise, vibration or harshness as the price point of the vehicle allowed to be spent on satisfying the requirements that ultimately come down to making the car acceptable to the customer base.
There is a big difference in a Caprice--a "family sedan" and a Corvette--a high performance sports car, for example, with different design and construction methods and criteria. Easy to see and understand.
The same is true to a lesser degree for a Caprice/Impala SS sedan and the 9C1 variant of the same vehicle. Spring & shock characteristics (stiffer or softer, valving design, different rate or load rating), tires (aspect ratio, tread design, recommended pressure), wheel size & width, and the body bushings (selected by rubber durometer, internal design--such as the number of voids-- length of spacers and lower cushion contact gap) are all selected/combined to meet specific performance parameters.
The differences appear to be quite subtle, and there are so many variables, it would be quite a task to become familiar enough with every different combination on any particular vehicle unless you worked (for GM) in the vehicle development area and had a good working knowledge of how the testing is done that determines each combination of factors. The OEM parts bin is full of parts the engineers and production people had to choose from when developing and building this car.
In the case of the Impala SS vs 9C1, it is easy to understand that the intended vehicle customer and use was distinctly different, enough so that the body mounting system got some special attention. Whether or not it was by accident or design, the Impala SS ended up with what most of us here on Forum seem to believe is an "inferior" combination of parts. Add to that the fact that we are now dealing with parts that have been in use for 8-10 years and in many cases are badly deteriorated. As enthusiasts, most of us will be of the mindset that we want the "best" on our cars, in whatever area (we) choose to focus, and the biggest factor is often whether the change is within financial reason. When we start modifying the car--engine mods, exhaust mods, changes to suspension, tire/wheel sizes and sidewall heights, powerful stereo systems, etc, we totally disrupt the OEM's tuning of the body mounting system as far as what will be felt in the driver's seat & passenger compartment.
So, it becomes a matter of whether the OEM has a good set of parts that can be identified as "best" or whether any aftermarket method may be superior--it will depend on what can be withstood in the driver's seat, and whether the specific use of the car will warrant the use of parts that impart a much more direct path for NVH "inputs" to the vehicle operator.
To answer your question, there IS a limit, but I really have no idea how a B-body would "behave" with hard plastic in place of the rubber mounts, but I do think you could notice the difference.
One thing to realize is that the factory parts may not always be available into the future, so if you have any notion that you want to maintain the OEM mounts as the vehicle ages, you should try to get your hands on those pieces now, even if you don't plan to install them for another 5 years.