Originally Posted by Razathorn

The kit arms are "gull winged", which puts the shock lower than the 521 flat arms. The effect is that the progression of damping and spring is different between the two arms. The gull winged kit arms take longer to "stiffen up" during compression, where as the flat arms do not. The effect is that damping and spring is far softer initially on the kit arms, which is similar to running your shocks way further in, or a much lower rear roll center.

Tebo told me that with the kit arms you really need to run a thicker oil in the rear compared to the flat arms. He didn't tell me any of the stuff I wrote above -- that is MY take on what is happenin
The bottom line is the 521 arms roll less, and the classic outcome of that is the car squares up better out of corners and rotates a little more. It's like raising the rear roll center significantly more than you can with ball stud
washers. It puts the rear end in an entirely different roll profile. On most surfaces, even loose ones, it is better IMHO.