Anyone have experience? I know you can do this with normal wood stoves. The previous owner did stuff I need not explain but basically secured that only a 6” pipe can go through the cieling, but I want an 8” system. Will it work? How much vertical draw will I need for the transition to not throttle it and back draft?

If you search well, there is a subject about the exact same situation, may be two years old. Or more.

Planning on doing that this summer.
 Replacing my old standard double barrel stove in the shop with an 8" J tube and a double brick bell. It will have a 20' rise up to the roof with most of it being 8" but the roof has a 6" metal bestos chimney so reduce to 6" for the last 2'.
I know that Matt Walker and others have done this.  I expect/ hope for , no issues with it.

You would want a smooth, gradual reduction to reduce the throttling effect. I would say that an 8" to 6" reducer 6" long would probably be enough.

This thread describes a J type rocket where the flu goes down from a 8" to 5" before it exits the house:
http://donkey32.proboards.com/thread/2439/tell-burning-clean