GSRs run 15.7 stock, so your LS is already slower than that stock. I don't know about 17's though. That does sound a little too slow for your car. How many miles are on it? How's the engine running? Not changing the oil often enough, and not doing regular maintenance can be making your engine 'unhealthy'. You should take care of things like oil change, tranny fluid, fuel filter, check your fuel injectors, water pump, all of your belts, etc. Take care of those things first. Then when you are sure your engine is running good, start with your bolt on's. What kind of exhaust do you have? What kind of cai do you have? Most exhausts and cold air intakes aren't really gonna drop your ET a whole lot. Wires and plugs don't change your time either. It takes more to be fast than people think. But if you are running 17's now, when you drop to 15's, that will be signifigant to your car, but still isn't very fast. A full exhaust will help you out. Header, cat, and muffler. DC sports are most reccomended, but even they only give you about 3-4 hp gain. Powercore gives you the most, about 7-8, but they run around $425 or more. A good muffler can help out more than a cheap one, but they do cost. I have a Tanabe on my del sol, and it's one of the best flowing mufflers you can get, but it did cost about $500, with the piping and installation.
If you want a quick fix, nitrous is best for that. A good setup can drop you almost 2 seconds. And you can get most kits at around $500. I would recommend Nitrous Express if you know what you are doing, or have someone that can install it for you (10 hour install time). If you want to do it yourself, get ZEX. It's easy to install and has the computer that won't let you run too rich or too lean.
If you want to stay naturally aspirated, go with some pulleys, cam gears, better injectors, fuel rail, ets. All these things will make small differences, but together they will help out.
If you really want to go fast, start thinking about engine swap. Or an LS VTEC setup. Those produce good hp and torque. And then spray
