I'd ward you against the Y setup. Yeah, it looks cool, but its an overall loss. you'll be dividing the same pressure air into two mufflers when you can get by with only one. you'll have that one muffler workin harder syndrome too unless you split perfectly in the middle. plus an extra muffler means extra weight, so you'll end up in the same situation as before...either:relatively small gain, no gain, or loss of hp. plus you have to deal with backpressure issues, which, on the gt, aren't so difficult to deal with. do what you want though. it really does look cool to have dual exhaust on our cars, but it does depend on what you're going for as well: fast and furious or legitimate speed mod.
c-man: thanks for the info!