I have the Stillen Engine Damper fitted. http://www.stillen.com/product.asp?id=S ... c=PE&m=all
I couldn't give you a before and after comparison as it was fitted when I got the car, but with it fitted the engine moves very little when giving it a rev, so I assume its doing its job. In-fact the whole car rocks instead!
It is a nice, well made item. It is mounted so it get direct linear force through it rather than a bit of lateral like the ones that mount to your top mount.
The advantage of these Engine dampers over harder mounts is that noise and vibration transfer is much less and they are much easier to fit than new mounts.
Reading reviews from people with them fitted, they generally report improved throttle response along with the car feeling more stable under hard acceleration, especially when exiting corners. That would also agree with what the previous owner told me he found.
Fitting instructions here; http://www.stillen.com/product_files/307355~inst.pdf