Most efficient mod for $$$$$$

[Pat]

Definitely weight loss. Like he said, it's free, and improves accelleration, braking and cornering, not to mention gas milage and tire wear.
Of course, you could also put in some Pagid Sport Blue brake pads. That'll give you better braking for little money, assuming you have good tires.
Or, if you're nearing the end of your tire life, you could just get better tires than what you normally would. That can make a huge difference.
If you're looking for more power.....well, there's not a whole lot you can do that'll make a big difference in power without spending a bunch.