I've been burning with the 6" pipe. My only problem with it has been a possible overdraft issue. I'm not an expert, but I do wonder if having an 8" pipe would only make this worse to be honest. My (and my installer's) read of the manual is/was that the "operating with doors open" statement references actually leaving the doors open with a fire for viewing and running, not for loading. Thus, we did not read it as "don't open the doors with a fire in there if you have a 6 inch pipe". Now that I have learned how to properly operate the stove, I have no problems with smoke in the house - I now know how to build a fire that starts up well without having to keep opening the doors to fan the flames etc (I use a modified top-down approach), and I make sure I open the bypass damper a few seconds prior to opening the doors etc...
I am sure every install is unique and your mileage may vary - Perhaps there are places (high altitude?) where even without the desire to use the stove like a fireplace one would need the 8" pipe. I don't think my install is one of them and perhaps OP's may be another. I reviewed my planned usage and figured I would never want to go into 'negative efficiency mode' of the open fireplace so chose to go 6" and am not sorry for the cost savings (of course I'm not even fully through my first season so what sort of expert does that make me? Ha!).
Back to making my own mistakes now...