Are readings of more than 90°C (194°F) for FB-DIM modules not a bit high?

No, temperatures in that range are normal for FB-DIMMs and are no cause for concern. Each FB-DIMM contains its own controller, called Advanced Memory Buffer (AMB). An AMB also contains an internal sensor which measures temperature directly inside the chip. The specified maximum temperature range of an FB-DIMM, measured by the AMB, usually lies in the interval between 95°C and 125°C (203°F .. 257°F). It will vary from manufacturer to manufacturer, a typical value is 110°C (230°F). If you like to know the exact specifications for your FB-DIM modules, note the part number and manufacturer of the respective modules (printed on the label of each module), and download the data sheets for those parts from the manufacturer. Nearly all chip manufacturers and RAM vendors publish the specifications at their web sites.