woodstove Chimney sizes

In a continual attempt to use what I have rather than buy new stuff, I have a little puzzle.

I have a simple cast iron box-type stove with a 6" diameter hole at the top.

I have a batch (3 3ft sections) of 8" triple wall chimney for outside the house.  

I can get all I need of 6" single wall for inside the house.

QUESTION:  does anyone think I'll run into trouble with my draft if I have about 9 feet of 6" that then flares out to maybe 9 feet of 8" pipe?  

that is, it's going from a good run of smaller tube to a good run of fatter tube.   all straight line, no turns.  

That will cause a reduction in the velocity of the exhaust.
In winter temps, when you would want it to work its best, that reduction in velocity could cause down draft issues.