Turbo.
That's the cheapest way to get noticable performance. Our cars aren't made to go fast in a straight line.
You could do an intake, exhaust, pullies, etc. but, you may only gain a minor increase. Now, if you had a turbo setup with even just 6psi, you'd notice a significant increase. Add the previously mentioned supporting mods to that turbo, and you'll have a completely different car.
If you don't have the $$$ to spend, you might be best off just upgrading the suspension and leaving cars behind in the twisties