Differences between an overpipe/midpipe/downpipe?
 

Sorry for the question just a bit confused.

On the NP website they have a downpipe with cat, then they have a downpipe with cat that replaces the overpipe.

What is the difference?

The downpipe with cat that replaces the overpipe is straighter and not a direct replacement. The stock piece is made the way it's made so that it clears RHD steering racks; we in the LHD world don't need a replacement for that pipe that's the same shape because we have no interference there. Engineering a pipe that's straighter and replaces the stock overpipe not only makes more power, but also gets rid of one gasket.