What will they do for me? I saw a set 4sale on the bay not sure what they would do over the stock rubber mounts

 

Do a search -- discussed in a couple of threads.

Quick answer:

Upside: makes car feel like it was made as a single ingot rather than a bunch of parts screwed together -- improvement in drivetrain and shifting.

Downside: no give at all (compared to stock hydro-elastic), so you can get a "wham" on road bumps/irregularities and in cold weather can have some minor buzzing or vibration from interior parts if anything is loose.

Dpends on how hardcore you are and what your comfort/performance equation is -- I already had PSS9s on agressive setting and GT3 sways and agency links, so why not go whole hog. If being sold used on e-bay, seller may not have had the tolerance.

Best are factory RS solid mounts available from SunCoast for $400 http://www.suncoastparts.com/product/PKRSMOUNTKIT.html?Category_Code=996performance

 

Huge difference on feel and especially, shifting. As jr2 mentioned, it will make you feel absolutely everything. If you don't mind about being "hardcore", it is for you. If you still enjoy your heated seats, listening to air supply and complain how the cars are a little stiff, definitely stay far far away from them.

 

You might want to try semi-solid mounts first. I know some folks here who have installed them and have been happy with the results.

http://www.wevo.com/Products/Chassis...gineMounts.htm