1. Root yung stock 2.3.4 (pag rooted ba, ma-uupdate ko parin ito to official SE ICS release?)
If SE will release the upgrade for Android 4, even you have a rooted phone still you can upgrade to the latest OS.
2. Custom Kernel (Doomlord) - for overclocking
I have no experience in OCing a phone though there are rooted apps that can be use to set your phone clock speed.
3. Apps2SD or Link2SD all games and media to SD card default install, free up at least 250mb internal.
You can install app2sd once you are rooted. I am not sure but in Samsung you can move your apps to SD.
4. Chainfire3d, BraviaEngine, Xperia Camera mod, Go Launcher - installed sa internal.
Chainfire3D supports XPLAY?
Somehow I think you need a full custom ROM, why dont you try CyanogenMOD 7 - Team Xperia Built. Also there are some strip down custom ROM from XDA.