We did our first ever trip in our Davis 14x16 tent with the larger 'Peak' size stove a few weeks ago. First night there, it got down to 10-15. We had to stoke the fire about every 2 - 2.5 hours. I attribute that to burning soft wood (all that we could find in our area) instead of hardwood. Other nights, it only got down to barely freezing and we only had to stoke it every 3.5-4 hours (so twice a night, really). I think if we had had hardwood, it would have been fine to go all night on the 'warm' nights and only need stoking a couple of times on the 'cold' nights.
I actually liked having the water tank on the side. We would fill it up before going to bed and it acted like a humidifier keeping moisture in the air so we weren't waking up with bloody boogers the next morning from the dry air. It was also nice to have hot water on-demand for food cleanup or just to wash your face off after squinting in the wind/dust all day and it left the top of the stove clear for warming up food if needed.
As far as seal goes - we haven't modified ours at all (it comes with pipe damper) and we've only used it twice now, so maybe it will loosen up after some more usage, but if we close the vents off, there would be unburned wood left in the stove later, so it was sealing tight enough to actually snuff a burning fire out.