15" wheels are the devil.
Are they fuck. I was very happy with how my car behaved on 15s.
I think its more a case of the correct setup for the wheels used, when retro fitting any non stanard wheel things like suspension geometory/rolling diameter of tire used/offset of wheel/width of rim/weight of wheel & tire/plus weight of car itself, all play a major part of the equation.
It too easy to fit cheap wheel/tire combo's to match a budget(or image) and end up screwing up a good well balace chassie.