No, you do not need an image stabilized lens to pan. Subject blur is caused by panning at a different speed ("angular velocity") than your subject was moving when you pressed the shutter. If the background blur for those shots was what you wanted, then my only suggestion is to practice...a lot.
Panning race cars is one of those times when a 6-8 FPS frame rate is useful. It won't make your car any less blurry, but it will allow you to get shots of the car approaching, broadside and moving away, all in the same sequence. I greatly prefer the broadside view. One thing that I have found helpful is to position your body so that you are most comfortable at the end of your shot sequence, then twist your body, with your feet fixed, to where you will begin the sequence. Otherwise, there is a tendency to slow down as you shoot your sequence.
As for shutter speeds, when i try to pan race cars going well over 100 MPH, I use a shutter speed of 1/60 to 1/250 second, with 1/125 being the most common. Even then, only 5-10% of my shots are acceptable.
Hope that helps