1) Restrictor pill - used by factory/stock set up to achieve 14.9 psi (or there abouts). It is installed in vacuum line between turbo and the turbos wastegate. If installing a boost controller, the boost controller removes the need for a restrictor pill since it will be controlling the boost. Should be plenty of stuff on google about this and how it works.

2) Clutch - I used to have the factory Subaru clutch on my WRX and it shuddered on take off (from a stand still) 50% of the time. I now have a Stage2 Competition Clutch and there is no shudder and it engages smoothly. The friction point is about half way up the pedal travel.
VisionR.com.au - Details : Clutches - Clutch Kit Stage 2 Sports Heavy Duty