I think the thermostat will not lower your engine temp. It will open sooner and will take longer to reach the operating temperature but after running for a couple of miles it will be the same.
105C is normal in that engine because at that temperature the high speed aux fan will start.
If you want to modify something, you have to deal with those aux fans starting at a lower temp. Read this 17 and 18 here:
http://pages.prodigy.net/jforgione/MB_S500.html