I can share my experience with you on this type of venting. My Quad Castile insert was installed in my fireplace and I installed a 1/8 inch thick aluminum block off plate to seal the damper area, it had a hole cut in it for the 3 inch flex pipe, and we packed the damper area with fiberglas insulation then caulked the edges to seal it tight. The installer ran the flex vent up into the chimeny about 6 feet.
This seemed to work ok, my insert ran ok, but I did notice that start ups were a little rough sometimes, in the real cold weather, the insert would often give a hefty " thump" after struggling to ignite with a lot of smoke in the firebox, and the draft didnt seem to be as good as I thought it should be because my flame never really danced around or was a whiteish color as I had seen in the showroom at the dealers. Occasionally when it was very cold I would also get a whiff of smoke in the room on startups, but I could never find a leak anywhere behind the insert, there was only 2 joints and they were tightly sealed with silicone.
After that season, I was talking with the dealer and I mentioned these issues to him, and he suggested that we install the vent pipe all the way up to the top of the chimney instead of just having the pipe in there part way. He said that often times when the chimney is very cold that cold air is so heavy it wants to drop down, and the combustion blower has to work a lot harder to overcome the density of this cold air and push its way up and out. Once the fire is going for a while the air in the chimney is warm and of course hot air rises and it flows better. So, we removed the short pipe, and put in one long one about 17 foot and terminated the top of the chimney with a stainless sheet bent all around and sealed tight with silicone, and a rain bonnet was put on the top. Prior to this all I had on top my chimney was the galvanized metal bonnet with screen to keep birds out of the fireplace. So now I have a tight seal at the top, and one long vent all the way down to the insert, and the damper area is sealed tight. No more large volume of dense cold air trying to drop down. The volume of the 3 inch flex is a lot less than the old 8 inch liner.
The difference was amazing. I have a much faster start up with almost no smoking or very little, and the flame is much more active, it swirls around the firepot and dances about nicely as it should, no lazy flame at all now, and there is never any odor at all when it ignites.
I suppose I could have left the short pipe in there as it did work, and once the chimney was hot inside the draft did improve but this new vent setup is so much better.
My manual shows several choices of venting, and using a short section of flex pipe into an existing fireplace chimney is one of those but, I dont think its that great, and I do know for a fact that going all the way up with the flex pipe worked a whole lot better.
Hope this helps you in your decision, its a little more money for the longer vent but its well worth the performance in the long run.