Had the same issue -vehicle would take too long to warm up in winter (compared to the past). I also got DME error 2EF4/82 (https://www.drive2.com/l/535096851062325630/). I replaced the thermostat and dual coolant temp sensor, refreshed coolant and bled the system. The vehicle now warms up as normal and cabin vents now have a nice warm air flow when set to 72F. The error code is also gone. I hear that the X5 t-stat is designed to fail open, so it won't cause an overheat condition, but if I were you I wouldn't wait to prove if that is indeed true.

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'05 E53 X5 4.4i, '97 E39 528, '07 E92 335i, '16 F86 X6M.