About the injectors, The STI uses Side feed and the WRX uses the top feed. If you are just replacing the short-block, you will use the top feed injectors. The fuel rails are what determines the injectors, as well as the pig tail on the wiring harness.
I am running a Crawford S2 (2.5L STI block, STI forged Crank, STI Forged Rods, with CP Forged Pistons) and WRX Heads. The compression can be pulled down with the right set of pistons. I am running the WRX heads setup from CP which with the STI head gaskets, gives me an 8.1:1 compression ratio. On top of this I am running the FPGreen, which makes for one torque monster. I am considering going to a GT35R within a few months, once I finish building the Transmission that I blew a few months back.
If I can help anyone out with anything, feel free to let me know.