The exhaust system is broken down into a few different parts. Axle Back meaning the piping stops at the rear axle while the mid pipe (b pipe) connects from the rear axle to the downpipe. The downpipe is where your cat sits which in turn connects to your header...then yo block.
As far as performance yes the full cat-back would be best for full performance. The exhaust piping is bigger on the aftermarket products so you would want a continuous piping diameter for optimal flow and power. Hope this helps.
GD3blaze07'