Well, I like most HKS stuff, but not their downpipe. The reason is that it is not a bellmouth or divorced wastegate design. It doesn't give you any gains except for noise. The TurboXs (and most all others on the market) are. However, you can't use the TurboXS downpipe with any exhaust except theirs. I would reccommend the Invidia downpipe.
I haven't seen one of those exhausts up close, so I can't say for sure. If it is 3" at the flange where it joins the downpipe, it will work fine. If it tapers to 2¼" like the stock one, you might have to use some RTV sealant to prevent leaks.