There are lots of good budget coilovers that'll let you adjust ride height. Feal and Fortune come to mind. I've heard good things about the BC, but havent done too much research on those. To get some extra slammed look you can do the RCE lowering camber plates.
But please keep in mind, that the suspension of the STI really starts to suck when its super low. You *CAN* fix that with some really expensive redesigned suspension components, but its pretty much only worth it if you're in some crazy tarmac track class.
You can also bag it and get crazy low and then pump it up if you need to clear the speedbump... (I think there was a journal of a bagged guy on here. Let me try to find it)