I would suggest upgrading the heat exchanger to the frozenboost front mount which is a very simple install and not only looks awesome but really helps a lot with cooling. I also swapped my spark plugs to GTR ones which are also recommended but don't come with the Stillen kit. I dont feel the need to upgrade injectors either since you will probably want to stay with the 8psi pulley and I wouldn't recommend swapping the impeller unless you plan to upgrade injectors/UPREV GT MAF along with it but at that point comes the higher risk and expense of reliability which will ultimately be based on the tuning. I personally wanted to keep it as safe as possible so I did the following.
1. Stillen SC with 8psi pulley (Had the 9psi but went back to 8)
2. Turbosmart Dual Port BOV (For Sound
wanted to replace the standard Bosch unit)
3. Frozenboost Heat Exchanger (Added Cooling)
4. Stillen 25 row oil cooler (Cooling)
5. GTR Spark plugs
6. Stock cats per Stillen's recommendation (Had FI RHFC's but went back to stock cats)
7. Custom Uprev tune (Highly recommended)
Since you're auto I would advise to also get a tranny cooler.