OK, we'll assume the connector pipe goes into Class A Chimney at the ceiling (called the chimney support box) Wanted to make sure you had a system that would work first.
You have an insulated chimney, which is the best, BUT you need to check to see what kind of insulated chimney it is. (If you don't have the paperwork from it, you can take a section apart near the top, you can see where they go together what you have) There are two types. "Pack" which is stainless inside pipe with a dense heavy insulation between the inner and outside pipe. The other type is triple wall insulated, which is a stainless inner pipe with a second larger pipe, and finally a third outer pipe on the outside. Some will have only air between all the pipes and they cool the inner pipe so well, they run too cool and load up with creosote.
The only triple wall pipe that should be used with your stove will have an insulation blanket wrapped around the inner pipe between the first and second pipe. This keeps the inner pipe hotter to prevent a large build up. The object is to keep inside flue temperature above 250* all the way to the top. So by monitoring the pipe temp just below the start of chimney, you know about what temp you have going into the insulated part of the system, and should not lose too much all the way up. If you try to keep the temp at 350, you can assume it may cool to 250 at the top. This is where you will get the greatest accumulation, at the coolest part. Time will tell as you check it frequently if you are running too cool.
Now you know what temps are ideal. Without any baffle, there can be so much heat lost up the chimney, the exhaust out the top can be far more than needed, so consider that lost heat that brings down the efficiency. Closing the damper partially slows down the exhaust flow, and also slows the air coming into stove, so it slows the burn as well. Slowing the velocity does make a difference in speed up the stack, but it doesn't make much of a difference in temperature until the fire slows due to slowing the incoming air down. The baffle plate directs flame to the stove top instead of letting it go straight up the pipe. The design of the baffle is what determines how much heat is left up, or soaks into the stove. Since you never ran the stove without a baffle, you may just be running too cool by not opening the air vents enough. (or closing the damper too much) See if you can maintain 300 to 350 mid burn cycle with your baffle style. It's an untested design, and not one used by Fisher.
Here is a factory "Smoke Shelf Baffle" that was used in later stoves the width of yours;
View attachment 123814
The square inches across the outlet pipe of your stove (8 inch round) would be 50.24 inches, so with a 8 inch pipe and chimney, you would need at least 50.24 square inches of space for the smoke to exit stove. This is "smoke space". Reduced down to 6 inch flue, you would need at least 28.25 square inches of smoke space. In other words, you can't make the space the exhaust need to travel smaller than the size going up. It will smoke when you open the doors, and not allow enough heat up to stay clean.
Does yours smoke inside when you open the doors slowly ?
This is the first sign of a exhaust restriction inside or outside of the stove.
Notice the baffle shown above closes off the sides and you can measure the opening easily. With yours, you will need to measure the opening for smoke to travel out, and figure the square inches you have by multiplying L X W of opening and adding each side together. The design looks like it would roll the smoke forward and maybe work well with doors closed, but my guess is that it may roll the smoke right out the door when they are open.
Off topic; is that a UL listed stove board under the stove, or a homemade tile or stone protector?
