I had solid mounts on my 79 TA and you can feel the difference. It feels just like you have something up against the body that shouldn't be.... you get more vibration from the engine. The body mounts do serve a purpose quite well. That is, if you haven't eliminated them with solid mounts..... It's really easy to picture what they do. And what they can't do if they are solid. The bottom line is; Expect some extra vibration with solid body mounts.
I've talked to (and my personal experience) is quite the opposite with solid mounts. Plus, when things like mounts are designed they are designed to absorb vibrations of a particular amplitude and frequency. Once you change the material from rubber to poly you affect the frequency range for the damping. So it doesn't work the same as saying "well it's softer therefore you get a little more damping."
If you have a problem with solid mounts either you need to get better engine mounts (if it's engine related) or better shocks (if it's road input related). Those are the things that should be doing the damping, not your subframe.
The type of polygraphite that was formulated to make subframe mounts was designed to absorb the same vibrations that the original rubber was. The only difference is less 'flex'. It's the 'perfect mount'. Just as solid as solid mounts but with the dampening capibility.... My 79 TA that had the solid mounts, and extra vibration due to them, was a frame off restore with attention paid to detail. The only mistake made was the solid mounts. The new owner agreed and has already switched them back to polygraphite style 'dampening' mount with great results. The odd vibration is now gone.
I'd love to see the data that shows these mounts of different material absorb the sample frequency range amplitude levels.
As for switching between the two, there are plenty of studies where some participants that received the placebo pills (sugar pills) instead of the real medicine somehow still noted an improvement.
If you look at the link I posted earlier there are plenty of people running solids (on 2nd Gens even) without this affect.
The bottom line is; Expect some extra vibration with solid body mounts. 
