The 3.5SE and 2.5 exhaust systems are different, they are not interchangable. On a 2.5 there is no "y-pipe".
Honestly, the best exhausts that are out there for the 2.5 is the Greddy Spectrum Elite Cat Back (Single muffler, dual outlet setup) and the Nismo.
There are a few others, if you search though

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