I helped a buddy turbocharge his celica 5sfe motor 10 or so years ago. We left the motor alone and it boosted up to 8 psi fine. One of the most important mods is colder spark plugs gapped smaller. If you keep the stock plug setup the motor will tend to cut out due to the spark getting blown out. But the biggest problem is overheating the stock plug causing it to come apart and in the process ruin your motor.
Spark plug technology is so simple and often overlooked but very important. Here is a good website to read up on....
http://www.ngksparkplugs.com/techinf...31000&country=