I'm with Rob on this one. I'd stick with the 8" pipe (for the moment) and extend it at least a section (4') first off. That's the easiest and most practical thing to try. If it works, your off to the races. If not, then you may have to spend the bucks and convert to 6". I understand what that (wily) old farmer Fred is saying about design, but in the past many stoves were designed with a standard 5" or 6" ( some furnaces up to 8") pipe outlets that exhausted into larger masonry chimneys well before the advent of matching sized stainless chimney.
Is a matching flue to the size of the stove outlet better? Probably it is, but you have 8" which isn't out of the question larger, and putting an extention on to test, is the easiest way to find out if it solves the problem. JMHO.