The M57 thermostat is nominally ~88–89 °C, and in normal ambient conditions the engine will typically stabilize in the high-80s to low-90s °C range.

In my case (−15 °C ambient), after ~25 minutes and ~9 miles of mixed driving, coolant is ~75 °C, then rises to ~80 °C shortly after sustained 55 mph. When ambient is near freezing, it readily reaches ~85 °C+.

More importantly, the overall thermal behavior is coherent:
- DPF regenerations occur normally and raise coolant into the low-90s °C regardless of ambient temperature
- Cabin heat is available quickly even in deep cold
- DPF delta-P scales normally with load
- Fuel consumption remains typical for conditions
- No under-temperature faults

That tells me the engine and aftertreatment temperature strategy is functioning as designed. Partially blocking the radiator might raise steady-state coolant temperature slightly, but on a modern emissions-controlled diesel it also alters charge-air, oil, EGT, fan, and regeneration behavior. It becomes a moving target depending on ambient temperature, vehicle speed, and whether a regeneration is active.
Since the system is already meeting its operating objectives, I don’t see a clear benefit in manually overriding airflow.

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8/2011 X5 xDrive35d Sport, Black Sapphire Metallic, Black Nevada Leather, Fine Burr Walnut Trim
2/2001 M5, Jet Black, Exclusive Complete Black Walk Nappa Point Heritage Leather, Black Cubic Trim