I hope it wasn't the S-tech springs, you're going to be replacing your struts in no time at all.
You have quite a bit of reading to do my friend. If you're looking for 300 WHP, you're going to want to look elsewhere for a turbo. You can likely get there with a an agressive tune and some other supporting mods, but if you want to do it easily without having to push the limits of the turbo, you're going to want to get a 20G, or HTA68. Granted, they're bigger and badder, but they both have spool characteristics of stock and you don't sacrifice much by way of drivability.
You're going to need bigger injectors (750's at least), bigger TMIC, fuel pump (Walbro 255 lph) and downpipe, to get to this goal, amongs the other things like boost gauge, wideband O2 sensor with gauge, and others.
Also, the AccessPort is fine and dandy for their OTS maps, and it also opens the door for tuners, but you could save yourself a lot of money with Open Source, but then it comes down to how serious you are about learning about your car and monitoring things.
Oh, and welcome to the site.