@M0GLO I'm under the impression that it's an intrinsic limitation of the LimeSDR. I can't imagine that the USB would make a difference.
Also, I understand that lower frequencies can be 'up-converted', then run through the FPGA, and 'down converted' on the other side? Higher frequencies would be given the opposite treatment. This way a frequency of eg 791 MHz could be run through the gadget in parallel with a frequency of eg 862 MHz. (862 - 791 = 71 MHz).
I could be wrong!