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 bottom line is; Expect some extra vibration with solid body mounts. 
