Couple of general comments:
1. TStat would be my first choice ... it may seem weird, but lots of folks how have the coolant system "opened" (fluid drained out) seem to have problems with tstats when the system is resealed and filled - is there a failure mode?
2. Type of coolant is not likely to make a major difference in temperature. It will, long term, likely mean some problems.
3. Proper ratio of coolant to water is likely a bigger factor in cooling efficiency than type of coolant.
4. Is there a failure mode for the viscous clutches during R&R& i.e. are they stored in a manner that causes problems? I recall my new one had a warning to store vertically vice laying them flat ... on the other hand they were shipped flat?
5. Within reason, I can't imagine a minor vane difference being enough to make a cooling problem.
5. Finally, there could simple issue of system not being pressurized. Start at the cap and work down through the system looking for leaks. Cap is about $5 ... if yours is more than 5 years old, give it a try.
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