some have stated it is 1" lower. Either way the point is if you buy springs that suggest a lowering amount of 2" for the zx3, then the svt will only get 1"-1.5" lower. I was told it was 3/4" difference.
I have the progress srings which are rated for the zx3. I like them. The ride comfort is similar to stock but it gives the car a pretty good clean lowered look and isn't extreme. the spring rates aren't that high which keeps the ride fairly stock feeling. The intrax springs lower the car more and having higher spring rates which will give it a stiffer/harsher ride or however you want to describe it.
both very good springs. with the intrax since they are higher rate springs you will have to replace the stock dampers sooner than you would with the progress.
you may also want to search the suspension forum. I know I posted pics of mine there, but due to the server issues they have had, those threads may have been lost.