Fortunately there's lots you can do. A set of aluminum GT-40X heads, a Trick Flow intake,manifold, and a E303 cam is a pretty common and potent setup. With the minor supporting mods like a CAI and exhaust. You could see gains of 60-80 HP, maybe even more with a dyno tune.
You want more power, but I'm guessing you don't wanna spend alot of money. Ik how you feel. Since you already have the exhaust, I would add a K&N cold air intake. This will increase your mileage by 1 mpg and will give u 15 more horsepower. Next I would get a performance chip. This will take out your governer and will recalibrate your stock chip. (It will allow more fuel in cylinders, faster throttle response, change shift points to allow your car to be the fastest with your engine. good chips are: PCD, Jet, HPC, Superchips, and Hypertech, all varying in prices) Next I would get new springs: make your car lower to the ground to reduce drag. It will make it look sweeter too. From there, assuming your not buying a turbo or super, I would get headers. This will connect well with your exhaust. The intake will be bringing in more air, and the headers and exhaust will be letting out more, reducing back pressure. Its harder to get more power after that. You can try slight changes like new motor mounts, or durastic like a new camshaft. The easiest way to make your car faster after that would be to start strippin off weight and getting rid of excess things. Hope this helped!!!
The stock heads is what is killing it. Put on a good set of heads, intake, cam, rear gears & drag radials and you will not recognize the car. Add forced induction on top of that with low boost and it will scare you.
The potential to go real fast is there because the aftermarket is endless for the 5.0. It depends on whether or not your wallet can take you where you are trying to go.