Ok I have a greddy downpipe, it's about 10 inchs long (including 90* bend) and at the bottom it is 2" diameter. The next part starts at 2.5" and slowly gets to 2.25" before the cat. Is this thing trash or useable?

 

2" is small. might work if you really need it.
for which turbo? and what exhaust you got?

 

18g turbo and 2.5" exhaust. If worse comes to worse and can use it very temporarly and then get a real 2.5 or 3 inch one welded up by an exhaust shop or something.