Smooth tubes and "Green filters". Probably $200 and 10+hp.
Hi flow cats will reduce the high side sill temps of the stock Catalytic Convertors and add 10-20 hp ($500). If allowed, No cats and oxygen sensor simulators.
For HP, 40% increase with just a used Roe Supercharger. Exhaust helps, but not necessary. Probably a 50% increase in torque starting at 2000rpm! Later, add exhaust, headers, and change pulleys to go to another couple hundred horsepower - if you want it.
Main advantage Roe compared to Paxton is not just the torque, but the ease of installation (6hrs DIY) and upgrading to higher hp. The Roe comes with a Vec 2 and is easily programmed with "smart cards" and has lots of after purchase help from Sean. Water/**** injection should be added after the miniumum boost (depending on pistons).
If you have a Gen 3 you'll have to go to a Paxton since there's no Roe available for them.
Ted