When the O2 sensors get really hot they tend to misbehave and the car can idle much higher because of it. At least, that's been our experience at the dragstrip. Greg said it's normal for the idle to be higher for a while until things cool down and they stop freaking out.
Of course, you happen to drive an NA so I can't see them getting that hot and if it STILL idles that high after leaving the track, that's a whole other issue. Hard to tell for sure from your description.
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