I don't consider decat and headers to be a "cat back exhaust" (catalytic converter and back), but they are definately a major part of your exhaust system. Many would say the most critical areas of exhaust power loss are the headers and cat/decat. However, for those *just* changing the cat-back, sound would be the principal reason, as few real power gains are realized with just a cat-back system. If you look at my signature, you will see the whole exhaust system was upgraded.
Storm titanium 2005 Elise with carbon fiber spoiler, BWR sway bar, BOE toe links, gPan2, LidBone, sector 111 wheel studs, Grey RAC Monolite wheels with toyo R1R, Team Dynamics 1.4 with Hoosier A6/R6s, Katana2 Supercharger, ECU with 265 flash. PPE headers w/decat, CEL angle eliminator, Silent touch catback, Rear Panel Eliminator, HKS intake, V2 fuel tank, Walbro255 fuel pump. HID lights.