Good question. I also wonder how much room is leftover/available to do more stuff with the FPGA?
Just came across this:
The FPGA is presently used to take care of things such as interfacing, digital down-conversion and tuning, and sample time-stamping.
http://fossi-foundation.org/gsoc17-ideas.html#limesdr-integrate-risc-v-core
FYI see the announcement here: