I doubt you will gain any power by installing a 4" exhaust and there might be a slight decrease in egt's, maybe. I have the 3.5" straight piped as well, with my ppe on level 10 I have gotten it up around 1200 after very long runs on the highway at WOT. I'm convinced that is sufficient, the only issue is the noise. It's pretty loud, I'm still kicking around the idea of installing a 5" system w/ a resonator. I'm not familiar with the BD ez amp, so I'm not sure what it does to your egt's. Some chips/tuners run hotter than others.
I don't know what kind of power the BD Ez Amp produces, but the stock straight pipe should be good enough. I would look into gauges just so you can keep an eye on your egt's just in case they get too high at WOT.
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