Yep ... I have a feeling that many folks have too much coolant in their systems. Second "feeling" is that many folks have less than a perfectly sealed system (bad cap, small hairline cracks in overflow, etc).
Seems like you can buy a cheap tool to check the coolant ratio ... can't remember, maybe it simply determines specific gravity of the material against a baseline.
Final thought would be that the viscous clutch could be contributing ... same issue as with the tstat - i.e. the curve for its activation is temp based and perhaps it simply is activating too late in the cycle?
1991 350 SDL (200K and she ain't bent, yet)
former 2002 E320 4Matic Wagon - good car
former 1985 300 CD - great car
former 1981 300 TD - good car
former 1972 280 SEL - not so good car
a couple of those diesel Rabbits ...40-45 mpg