I've been looking for some decent sport springs to run on my E46, since I can't budget for a decent coilover setup right now. Unfortunately they are all progressive rate, from what I have seen. I've heard that progressive rate springs aren't ideal for the track since they make transitions difficult, as the spring rate varies depending on the amount of compression. However, the E46fanatics guys swear by progressive springs and I guess SpecE30 runs them too.
Are progressive springs going to mess up my handling on the track?
You need to look at pictures of the springs installed. Some "progressive" springs are actually linear over their useful range. For instance, the springs I have on my Mustang are "progressive," but the progressive portion of them is actually a helper spring that is actually completely compressed when the vehicle's weight is on it. The actual useful portion of the spring is linear.
In the air:
With weight on them:
See? The usable range is linear.
7pilot
Reader
3/29/13 3:00 p.m.
I would echo sky_render's observations.
Unless you did the 2", 2.25" 2.5" springs used in coillover kits, I think you'd not be able to find any Linear rate standard appearance springs.
Even with the linear coil over set ups, you'll find that the really nice kits use at least a 2 stage set up with helper springs that may also function to keep the spring aligned and snug under droop.
m
When you see a coilover kit with progressive springs of any kind, they're being used as a cheap alternative to a helper spring setup.
In terms of handling they're not theoretically ideal but they seem to work OK.
I have used iboc progressive springs on my 924s for ever and really like them. No issues on the track at all
I used Eibach Pro Kit springs on a couple of my Saturns, and I didn't have any issues at the track. Then again, I was fairly new to track days and may not have been pushing them hard enough to run into trouble (though I certainly did at autocrosses 
).
This was about a 40mph sweeper. The car had Eibachs and a stiffer rear sway bar (I can't remember the numbers). You can see there's still plenty of body roll until the stiffer part of the spring kicks in, but once does it takes a good set and keeps on going.
In theory, if you're driving hard enough the only times you won't be in the stiffer part of the springs' compression should be at turn-in and track-out. Mid-corner handling should be entirely in the stiffer part. You'll get a bit more weight transfer as well due to the body roll - see the above photo. 
Like mentioned above - a lot of the performance kits with progressive spring rates are just a way to make a lowering spring that does not fall out when you jack up the rear of the car.
For a 90% street driven car I'd buy a set of H&R or Eibach sports (not race) to lower the CG a bit but remain soft for the road. Then buy a set of beefy sways with some adjustability to control the roll. Otherwise you will be knocking your teeth loose (well, in the pothole riddled NE anyway) all the time you are not on a race track.
progressives are ALWAYS progressives... the reason is the car is NOT stationary on its springs, it is always compressing, decompressing them.... So in essence you get changing rates 
Accelerating out of a right hand curve right front is uncompressed, left rear compressed, front left, and right rear are in varying stages of compressing, change the corner, or instead of accelerating, now braking... all the rates are changing all the time.
Do what YOU want... but the fastest cars on track will likely have linear rate springs
