ok here are the answers to your questions:

1) the s2k stock won't handle 12 PSI with any reliability. You'll have to have your motor worked on before going to that level of boost
2) A supercharged S2k will put out about 320 HP depending on the SC at 6PSI with a high degree of reliabilitity (IE Stock motor can handle it)
3) Final drive gears (4.57 + comptech reinforced Diff) will also help you catch your friends GTO at least 0-100. You may not beat him but you'll have more power in gears 1-4. This costs about $690 + install. I recommend reinforcing the Diff if you go FI. Youll probably also need a new clutch
4) FI S2k at 6 PSI should run low-mid 13s. 7-8 PSI won't move you into the low 12s but Gears + FI might.

Hope this helps you. The best model year to get is probably '03 because you get the 9K redline and 03 was the most polished of the 00-03 line (glass rear window, seperate tweeters, leather/metal shift nob) etc. If you want the additional torque and are ok with the lower redline than 05-06 would be the better way to go.

Enjoy!