Thus far I think Fast Intentions has the greatest proven gains and they run an x-pipe. Personally I think the Y pipe designs sucks and I think Nissan thinks so as well. Figure this, we all agree that the stock exhaust y pipe is very restrictive, but when Nissan developed the Nismo exhaust they dropped the y pipe design for an h pipe. If Nissan didn't think the y pipe was worth designed and maximizing further I can't see how it could be the "best."
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2009 Carbon Fiber/Red 370z aka Trish: Sports Package, Meisterschaft full catback, Fast Intentions Longtube Headers, Injen Long Tube Intakes, UpRev Tune: Sold 
2008 Lotus Exige S Club Racer aka Bella, always leaves me smiling
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