Low temp at a constant cruise could be worse than low temp after repeated booting - even cruising at 90 the engine won't be using nearly full power like when booted.
Just converted the V12 twin turbo AMG jobby, now starting on another AMG, 2007 6.3 V8 ML. The ML has a pump to help circulate coolant through the heater circuit, it's a neat little unit, probably only 2x2x2". Got me thinking these pumps might be handy in situations like yours or where coolant flow through reducer might be a bit low. Probably just a few quid from a scrapyard, looks like 19mm fittings, outlet is at 90deg to inlet, simple electricals are earth and +12v. Just spent an hour looking at the plumbing etc on the ML, had it running with climate at full cold AC on, then climate at full hot, then with climate system completely off - Whenever ignition is on this pump runs and water is circulated through the heater matrix, pipes stay very hot, so I reckon I'll plumb reducer in series with the matrix on this model.
Interesting to know! I always thought all but basic cars had heater valves to make the a.c side of heater more effective (because of no heat passing through). And more interesting is mercedes engineers need to fit a pump to help the flow through heater.
The v8 s type has a small coolant pump but still has a heater valves.
Today I had 2 short 90 mph runs which were one after the other, 15mins from cold start and 11c outside and both times the reducer dropped to 45c!
Should I worry about this?