DMAX, forgot to shift out of 3rd, must have been a late night stand up??
Yea, the issue with the electrically controlled stat you have be careful not prematurely condemn the stat because the DME may be artificially lowering the coolant temp at times without you being aware.
So at this point, I think city traffic and idle may be the way to monitor the temps?? But would having the AC on impact the stat heater? Well in the Winter I do not think many will have the AC on unless in defrost mode maybe? How does ambient outside temps influence the heated stat??
Again, I think the only way to really get a handle on the stat behavior is during the colder months, not a fun time to find out you need a stat, but may be the only way to really confirm it is not working correctly.
What I find is most stat seem to open too early, so maybe graphing the warm up cycle you would probably see the stat opening if you know what signature to look for?