I'm still trying to figure out the effect of this offset stuff and whatnot. All of this is hypothetical and I'm just trying to learn it so please bear with me. :help

Let's say the stock wheels are 18x8 ET20 and I want to replace them with a wheel that is 18x8.5 ET15. Am I still effectively giving the car a wider track even with the wider wheel?

Thanks guys. :D

I think track is measured from the centerline of each wheel/tire. So yes, your track would be increased by 10 mm, since you are moving each centerline out 5 mm. But the specific tire will probably have a greater handling impact than either the increased track or tire width.