Without the shim the piston has a float in it. A click in the compression. This will be noticed when getting on the gas into a high speed turn. The rear will give a little shimmy. Think of your rear end as elvis's hips. the car will give it's little shake then settle in. Normally it won't hurt the car. Two key points to this though. If you react to it and try to correct you can get out of shape fast. And also a bumpy high speed section could grab an edge and cause the car to get loose roll etc. Just food for thought