TBF anything that helps the car breathe easier will make improvements. Mod the stock airbox and stick a panel filter in it and a free-er flowing exhaust to compliment it. Sparks plugs and leads may do something, but as you say it's only an 8v, it's not designed as a performance car. With the money spent getting more power, and the extra insurance costs after declaring the mods, you may aswell get a 16v...