I didn't even know that they made springs. Like most SPT stuff, I am guessing there are better options in a similar price range. They obviously aren't too popular because they haven't been in any "what suspension mods should I do" or "what springs are best" threads.
They do make some decent springs, ratewise IIRC they're right around the Prodrive blues or a bit stiffer. With the MNSubaru discount we get from Morries, they're not a bad price either. What year and model are you looking for? That can dictate the need for other things to make them work properly.
I have heard the STI springs are the same rate with a slight drop compared to a more aggresive drop on the SPT.
If you've still got the stock shocks on an 03 they need to be replaced. That being said, the SPTs will work fine with stock struts in good working order. They'll benefit you even more with a better strut like the D-Specs. Any STI spring will work, but you'll need 2004+ tophats to use them. There isn't a drop ratio, there is only the amount of drop to a spring. The STI springs will drop the car less than an inch IIRC. More drop doesn't mean a spring is better, there's a lot more to it than just that.
This may be an option for you to save some money. http://www.mnsubaru.com/threads/fs-td-04-turbo-and-rce-springs.42727/#post-705749
