So, the CRS125 has a SFP+ port accordin to its specifications, and SFP+ can theoretically handle 10GbE connections.

Is that true for the CRS125?

Circumstances I'm contemplating:

1. Storage server on 10GbE connected to SFP+ port, storage clients on copper ports.
2. Two CRS125s to form a 48 port switch with a 10GbE backplane

...also, what difference does "high" versus "low" make under "SFP Rate Select"?

edit: routerboard.com has since been corrected to show the CRS125 as having an SFP port, not an SFP+ port.