Thought I would chime in, very new to post here but read the site a lot.
i bought the YS-640. It is truly a work of art, I think. I love my old offset smokers of years gone by - but, seem to value my sleeping all night more than I used to
It was 70F today, little to no wind. I set the pit for 180F. It equalized at 191F on the right and 187F on the left. Firepot is under the left end of the heat deflector.
Used a Maverick ET-73 and set the 2 probes at 4'' in from each end, and in the center fot he grate as measured front to rear.
4 degree differntial stayed constant for 2 hours. A couple of times the temps were 193-189F, but that is good enough for me.
I have never experienced this kind of stability. It is going to take me some time to explore all of the temp ranges this thing can give me.