You should be able to mix and match their SS parts and Ti parts. However unless you're gong full SS it really doesn't make much of a difference to go Ti on the muffler and SS on the rest. The sound won't be noticeably different and the weight savings isn't going to be significant enough where you'll feel it. GTHaust will run the same piping layout as stock for the most part.
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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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