I got my 3" direct from Milltek, having seen some of the crap peddled on ebay and the like, it was well worth the (way less than half a sportcat DP) money.
They will ONLY sell you one for a track/race car.
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Revo did me a track map to suit.
If I were doing it again, and on the cheap, I'd consider whipping the cerntrebox, pre-cat and cat off and having them rewelded. DP = power. Exhaust = sound.