Hi,
I have done a number of track days myself. The best mod I have done so far was to change the tyres to Michelin Pilot super sport tyres, reduced my laps times by a few seconds. My previous tyres were Continental sport contact three which were poor and melted after a few laps. The next best thing is to up-grade the brakes. It means you can carry more speed into bends. The last thing to do is increase the power of your engine, no point in doing it first if the rest of your car,especially if your tyres can't handle it.