Honestly, I haven't heard an exhaust on a 6 cylinder that I thought sounded good.... including my 4.0 SOHC ford and 4.0 I6 cherokee. My honest opinion is that performance gains would be minimal, and sound improvement is questionable (I haven't heard a 3.7L WK/XK with exhaust yet).

I know you have other mods planned. SO..... I recommend saving exhaust for last.