Yea I honestly wouldnt see why you could not buy a muffler and resonator and replicate the system. The only difference would be how you had the piping. If your doing this at an exhaust shop they are most likely going to use aluminized steel and its going to be press bent. If you want you could order stainless mandrel bends and have them weld together an exhaust that way though Im not sure how much they would charge you for labor. If you can get a Borla for under $350 then I would grab it, otherwise I would put it together myself.
I'm still in the market for a muffler/catback and Borla has come into the picture a few times, i've never actually heard it in person so I may still hold off for a while. We'll see... From the videos though they do sound good, and probably much better on the v6 compared to the 4cyl.
I have a Magnaflow catback. I know people with the Apex and IVIDIA or whatever else they have for the Eclipse. But in my opinion, Magnaflow and Borla sound sporty, but not ricey or raspy. Very subtle at low RPMs and powerful at higher acceleration.