For the price, especially if you're just going with the NM ENG. inserts its too inexpensive a mod not to try and it's one that doesn't involve scaring yourself with warranty issue. Do it. It's easy and the payoff is worth it. After a week I'd be surprised if you even notice whatever vibrations exist without having to consciously remember you mounted the inserts in the first place. My 2 cents.

 

My car is an auto, and the vibration went away above about 1500 rpm. The vibration was like the car dropped a cylinder and was felt in the floor. Even a passenger commented on a 2 mile drive to lunch. I put the inserts in (powerflex from WMW), and took them out after 2 days. I was told that some people had the vibration go away after some miles, so I put them in again for about 1000 miles. It didn't work for me, but like you said, it has worked for hundreds.

The vibration is less sensitive on manual transmissions, because the engine is under less load at idle. On my car, I have an accessport, and at that time the idle was a little unstable, which may have made the vibration worse.

Mike