I have to say that I am very pleased with the technical support I have received so far from MSI for my IM-GM45. Besides the MTRR issue I wrote about here, I had filed a second, minor request.
The second request regarded the shared video memory setting for the on-board graphics chipset. In the original BIOS the minimum amount of memory that could be allocated for video RAM was 32MB, which is way too much for the text mode that I need. So I filed a request asking for the possibility to select a lower amount of memory (preferably 1MB or less).
Two business days later I received a mail from the MSI support containing a BIOS with the fix for my specific request (fortunately it also contained the MTRR-fix).
This is admittedly much more than I expected. Thanks to MSI for the quick and helpful response to both of my requests.