There's a guy who has tested multiple springs and came back with crazy results, showing that Swift springs are about the best springs you can get, along with Hypercoils. The spring rate of s*** springs change during compression, giving you a very inconsistent spring rate. Not only that, but Swift springs usually have less coils than the bad brands.

http://www.northamericanmotoring.com/fo ... ynoed.html
http://forum.e46fanatics.com/showthread.php?t=775212

I also noticed that Swift sells a Thrust Sheet, which basically allows the spring to rotate instead of twist, which more evenly distributes force and reduces lateral load on the shock. I personally think that this just reduces sound and slightly improves spring compression, something you might think about with a full blown racecar to gain an small advantage, but not worth the cost in my eyes.

http://www.m3post.com/forums/showthread.php?t=504218

This isn't really a CA forum thing so I have to move it. I'm putting it in the Suspension forum, but I can put it in the 240sx forum if you would prefer.