You're paying for the resonators honestly, the Arks and the Motordyne Art Test pipes have weird, big arse resonators:
Ark:
ARK PERFORMANCE INC: Pre Orders for 370z/G37 R-SPEC Testpipes and Hi Flow Cats
I have some Megan resonated ones, and they are great... My 2 problems with them is that the O2 bungs are kinda short (I had mine extended and I put stainless steel wool inside them [and you can't extend them that much]), and the flange has one thread, so you use an OEM bolt to tighten that corner of the flange... Use a BIT of threadlocker, or tighten that biotch tight when you do it up