Hey guys thanks for all the tips. Thought I'd follow up and let you know what I ended up doing. I ended up using 12" spiral duct sleeving it with 14" to go through the roof. I went back to 12" before exiting the flashing. I used 4' of the 14" to make a low loss stack cap on the top. The 12" chimney and "super sucker hood" draws wonderfully, I'm really impressed. I was a little skeptical about the low loss stack cap but it works great in keeping the rain out. It does, however, let in a little snow.
Thanks again,
Robert