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allowe The gruppe-s is fantastic, never had a single problem! Also they don't come with an overpipe... But you can add one for just $75 (so why not at that point! If you ever decide to complete the whole exhaust it'll already be 2.5 inches to the frontpipe... Hard to get a good Overpipe for anything close to that.) AND it comes with all the gaskets and hardware you need to install both without a single worry. Great service, came quickly to my door and I'm still pleased. Not at all affiliated with the company, I'm just a fan of a good product. CNT and 1320 are just barely above eBay garbage, but they're crap anyway, I've known 4 now between the two companies that have developed cracked welds, these aren't really hard to fix: but there are definitely better options. The OFH is a good product, but compared to the Gruppe-S it's not worth the extra money IMHO. The OFH2 with the integrated Overpipe could produce a bit more power in theory, but I don't think they made many, and also $$$$...
What matters much more than the header is the tuning, I sure hope you don't plan to install a header without a tune because that's how you burn money without reason!
Edit: JDL is good too but I made almost the exact same WHP as my friend with a JDL (I made 202.4 and he made 201.3, both on the same dyno, both on an OFT OTS E85 UEL tune) and I spent less money. I also don't have an exhaust leak because everything I needed for fresh seals came with mine... He has developed an exhaust leak and needs new gaskets that he didn't think to buy at first. Not JDL's fault at all, but food for thought! JDL's have great build quality and service from what everyone I know has mentioned, so I'd say they're a good buy... But the Gruppe-S is just so perfect for the money.