Heres a tad of info as I have had many turbo cars:
The UPpipe is pre-turbo... this is what bolts to the headers
Headers --> uppipe --> turbo
Aftermarket uppipes are typically bigger to allow 20%-30% more flow (especially if the original Uppipe is catted). This allows the turbo to spool quicker (IF it is opened up on the hotside...) this brings us to the DOWNpipe
The Downpipe is connected to the HOT side of the turbo to the catback exhaust.
Turbo --> Downpipe --> exhaust
This is typically a restrictive pipe (also often catted) to cause backpressure on the turbo allowing the stock turbo wastegate to operate fully.
Aftermarket downpipes are typically larger to allow the exhaust gases to escape sooner, causing less backpressure and often resulting in better spool times (especially paired with an uppipe).
This is why catless exhausts are the only thing you see on turbo track cars. You have completely cleared any and all backpressure which allows the turbo to stay spooled at peak and also quicker spooling on acceleration.
For either of these products you most definitely should consider a tune, and have supporting modifications (less restrictive cat-back system and air intake for example) to let the car fully breathe
Hope this helps?