The ADC12D1800 board is not only for the RPi. The RPi could lock onto the FFT stream over Ethernet, or if programed the FPGA could send it's streams to another DSP server or over Ethernet your Qt-radio client running on a RPi or what ever... I dought the Pi is going to have a hard time running up against any hard hitters that run DSP servers processes large chunks of spectrum.
Broadband spectrum sampling has to take place on a custom FPGA based system, if its with GC6016 or ADC12D1800 or something else. Every one is bouncing off the walls on this RPi SDR thing.... There are lots of ways to set up 10 to 192khz sampling systems, the hacked usb tuner cards do about 2mhz, but to really get a SDR to Sample and synthesize RF with out specialized circuits for a one band or another.... I think we must tackle FPGA based SDRs.
The HiQSDR is a good example of a FPGA SDR, but If the design operated on the assumption that its going to feed a Qtradio interface or even stream to another DSP server and the size of the board could come down. http://www.hiqsdr.org/
The GHPSDR3 wiget group is discussing possible FPGA's to accomplish the task. If the board is developed to from specks from thou out the community, then we may be talking about a new line of SDR board for mid scale production.