For a fairly detailed debate about chassis bracing look here.
any one put a strut towerbar on an si?. Also even in post 84 on the thread you gave the designer even says it won't make your suspension stiffer. Just some food for thought.
Biggest improvment would be TIRES.
1. Tires
2. Proper alignment (not an OEM one!)
3. Dampers
4. Springs
5. Anti-roll bar or bars, depending on your springs.
06BlackSI you can IM me if you would like :)