Heres my 411 doing a lot of suspension work in Nissan Challenge and Corner3 Motorsports.
The stock springs that come on most coilovers basically are from one huge spring that they cut as it comes out. They then go and dyno the spring and if its in a certain % of like an 8k or 9k thats what it gets rated at and put in a bin and put on your coilover. So if you have cheap coils or something to be cost effective thats the spring you get. So between left and right you have a possible difference in rate between the two.
Consistency. Swift springs are very consistent. If you put a swift spring on the dyno the rate throughout compression is very consistent. Also if its an 8k or 9k etc you can trust and verify that it is actually an 8k or 9k
Swift also resists sagging so after a year of racing you arent pulling off a spring that is now an inch shorter or more from fatigue.
Material used on swift is very high grade hence the ability to resist fatigue sag and remain consistent
What this all means: You can actually improve the performance of an off the shelf damper by putting swift springs because you just removed one of the largest sub par part and now gain more consistency from spring alone. so instead of rolling around with springs on each corner that can all be slightly off marked rate all springing up and down slightly different from each other at different frequencies you now have 4 consistent performing springs around each corner.
http://www.corner3motorsports.com/pr...mm-or-70mm-id/