Stock air box with a K&N air filter will only get you 5hp, or so. If you have an "SSE" bonnie, you won't get any more "PSI" because you don't have a supercharger. If you have an SSEi, the supercharged model, you won't see a noticeable difference in boost output. If you have a */c motor, probably the easiest way, IMO, to get more boost would be to downsize the supercharger pulley. I would say a K&N is worth it, it'* a good beginers mod
If you keep your foot outta it, your MPG'* might go up a digit or two as well.