actually, the downpipe and catalytic convertor are one custom pipe on our cars. To keep it emissions legal, you would have to keep the factory downpipe/catalyst.
The downpipe is the tubing that goes from your exhaust manifold or header to the cat. the midpipe on most cars is usually the tube with the flexpipe and resonator on it, and is usually after the cat, or on the same pipe as the cat.
I think the XS engineering manifold will bolt right up to it, so if you plan to go turbo, go with that manifold and you'll be all good.
On a GT, you'll need a different manifold, or a custom downpipe to go from the turbo to your factory downpipe.
Header and cat-back should be more than enough for a N/A car, though.