3" Exhaust with 2.5" downpipe
 

Think it is worth any power or spool to use a 3" Cat and Cat-back with a 2.5" downpipe? Or should I just do both at the same time?

I am not 100% certain, but I would have to guess that it will absolutely help.

IIRC Faeflora has a 3.5" downpipe to a 4" exhaust. His is a little overboard for the power most people want to make, but you get the idea.

Haha yeah 4" would be total overkill for my 200hp turbo setup. I'm really thinking about building a 3" exhaust for now, and then upgrading to a 3" downpipe later.

I'd rather have a 3" downpipe and a 2.5" catback exhaust. The pressure you're trying to reduce is the pressure right behind the turbo, so you want the piping as big as possible there.

^why I will probably go low-mount when I get a snail. I would rather show it off with the hood closed anyway ;)

But the pressure at the turbo outlet is: atmosphere + backpressure exhaust + backpressure downpipe.

So the backpressures generated by exhaust and downpipe are equally important.

If you need to decide between DP and exhaust, replace the one with the longer path with something larger to start with, but definitely do both in 3" eventually.