Real-life Story: Raced Tofu (SS/TC) and lost. Found out he had the tire inflator kit instead of a spare tire. Took out my spare and then I beat him.
The spare weighs a ton for being such a small item.
If you're wanting something that's quick but still makes a small difference: Take out the back seats. I'm talking back and bottom cushion. It takes all of 15 minutes, the only tool you need is a flatblade screwdriver (for pushing the side tab to get the backs out). You get to hear the fuel pump, and you hear the turbo spool near the console (coming from the downpipe), you can hear the exhaust gases rushing through the downpipe into the catback, can hear the catback rumble better, and the sub sounds better. For extra points, you can remove the rear headrests. Takes two minutes and a rachet/wrench.
If you've done that: Remove the big bar that the seats hooked to. It weighs a lot and serves no purpose when the seats are out. Takes a rachet and five minutes.
Cost of said mods: $0
Benefits: Faster, couple city MPG gas savings, and a more exciting/fulfilling driving experience. Make sure to stick the spare back in if you're going out of town though. You'll thank yourself later. For around town, I've got a bike pump in the back. Weighs nothing and doesn't take much work to fill up the tires... and no need to stop and pay money to fill them up.
The main weight savings will be the spare and that rear seat bar.