I've gotten a 30-hp gain since I installed those door sill protectors. (Zoom zoom)
(I'm no help at all)
Anyway, in answer to your question, MelvynRay, although it doesn't add much hp, I would go with an intake, or maybe a cat-back exhaust, but I haven't really heard too much about Cat-backs for the tC. I know it's been discussed in a recent thread though.
The worst thing about it, is it has a crack-like effect on the driver. Once you feel the power, you can't get enough until you hit it with a shot so big you blow pieces all over the engine bay. Scary but fun!
Hey, has anybody tried or know someone who has tried this turbonator mod that they advertise on this site? They claim it adds up to 35 more Hp and gives your ride up to 20-25% better gas mileage. They're selling it for about $70. Now, I'm a pretty big skeptic and my knowledge about mods is pretty limited to say the least, but they do offer a 30 day money back guarantee. Any ideas, thoughts, or experiences you or anyone you know that has had with this would be appreciated. You can check it out at www.Turbonator.com.