Seems logical from a flow and weight standpoint not to split the exhaust and add all the extra piping like the stock setup.
I like the fabspeed design but a lot of the poor feedback prevents me from ordering from them.
I know this post is nearly 5 months old at this point but, I ordered and installed the Fabspeed turbo-back recently and had no issues with them or the quality of the unit.
I have a possible theory (which could ultimately be nonesense) why you don't see it more often:
After installing mine, there is very little room for such a big, single pipe to clear the space over the axle. I have spent a lot of time tweaking mine to get everything to line up in such a way as to not hit either the body or the axle itself, and still haven't gotten it 100% right. I have also heard reports of people who lower having complications. I have a mild suspicion that some of the other manufacturers don't want to have to deal with issues around this kind of fitment.
Also, outfits like AWE gain some flexibility with the dual-path, in which you can add/remove the resonator sections to make it "track" vs "touring", which is looking to be very difficult or even impossible with the single-pipe Fabspeed. (I know, because I'm looking to quiet mine down, particularly in Sport mode, and trying to figure out how to get a more robust silencer/muffler in there; there's not a lot of room to do a whole lot in this configuration without some like-significant fabrication and that would likely exceed the price of just buying another exhaust).
And lastly, I wonder if it's just about tone... this single-pipe has a rather deep sound, and while I like it, it's kind of... not Porsche-like. Not in a bad way, but sounds less like any Porsche I've owned or heard, including when I had the stock exhaust on here. Take that for what it's worth.