i'd say the main reason is, for a car purpose built for the track, the gains dont warrant the additional costs, the performance is practically even with the standard model.
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2011 Mustang 5.0 | 3.55's | Lethal Performance off road H-Pipe | Magnaflow Muffler Delete | JLT CAI/Bama tuned 417whp/ 404 ft/lbs
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