You might want to get a tune but you shouldn't feel a noticeable difference. I know some people say with the Y pipe you get more mid range grunt.
However, I believe the real reason is because you had potentially the lightest and most efficient flowing exhaust setup with the single muffler, now you've baffled it quite a lot. You may want to unhook your battery so the ecu resets, it is probably still compensating for the previous exhaust.
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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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