Most people replace the cat back section just for an improved sound, perhaps a few more horses (no ECU tune required). 'Best' and 'cheapest' don't go together in this case unfortunately. There are several good YouTube videos explaining quality and sound differences.
If you then add a freer-flowing turbo downpipe and mid section, you might see 20 more HP in total with the cat back. This will generally require an ECU tune.
