the Ford Parts sight seems to indicate that the SVT springs are the same as the sedan and ZX3... it looks like the struts are what makes the difference between a ZX3 and an SVT? That seems strange because I thought the SVT was an inch or so lower
they are the same in that they are interchangeable but the performance specs are not the same. they are 3/4" lower and have a slightly stiffer spring rate. the dampers are matched to handle the increased spring rate.
Less body roll=marginally
Best way I can describe it is the car feels tighter.It tracks better.You will just have a better butt feeling when you drive it.
I drove mine for a year with just SVT springs before saving up enough $$ for dampers & bar.I did'nt have any problems with stock dampers in that time frame.
I will say though,that when you can upgrade your dampers,you will notice a bigger difference than with just the springs.
With just SVT springs you get the 3/4" or so drop and then that general good feel about transitions and I personally think a better ride quality.It's firmer/stiffer without being jarring.
When you go with dampers(mine are Eibach Pro's),then you get major difference in feel.Much stiffer.Hitting expansion joints and sharp road height differences become jarring.
But that's the only downside.Cornering and transition exits are hugely better.No more mush on turn in and if you need to turn just after a dip or bump the car is on the ground and ready much faster.
I'm glad I did the springs first just so I now know the difference in the feel.
I would say do SVT spring swap in a heartbeat.
If you want a bigger drop or more spring rate later,they're not hard to change and I'm sure you'd have no problem moving them in classifieds.
Just the springs or springs/dampers,it's money well spent imho.