Thad:
Yes - when we switch from Tx to Rx it should switch from Interpolation to Decimation - Tim can fix that. (Thanks for reporting).
As for the Fstop question, today, the limit is the same (1/2 of the data rate).
Referring to the Block diagram:
https://github.com/analogdevicesinc/iio-oscilloscope/blob/master/block_diagrams/fmcomms2.svg
You are correct - for a decimation filter (Rx), the input into the FIR can be 2 or 4x the resulting output data rate. I would need to defer to Di about the limits of Fstop (how the filter design actually works) to know for sure.
-Robin
