With SC it has to do with exhaust pressure and our VVEL system.
It seems VVEL and GTM/Stillen SC on 8 or less PSI benefit from a stock cat as it holds the exhaust pressure just a bit longer. Now with VVEL control you could possibly achieve this through tuning and then open up the exhaust with TP so you have a true free flow exhaust but the exhaust cam is able to hold a bit longer for the needed pressure.
Now this is all with 8PSI or less, as you bumpt that PSI up I am unsure what VVEL will do and how stock cat versus TP will behave differently. I do know that aftermarket HFC do not work well with any system as they will break down faster & it will clog the pipe.
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