Are you sure HackRF really can send 20 MHz of bandwidth over USB 2.0 continously? Where did you find that info? (Seems I should try to push SDR manufacturers who use USB 2.0 to supply modes with higher sampling rates...)
I can't find any specific examples now, but most pages discussing the HackRF (including those from people who have a prototype) say there is no problem receiving 20Msps from the device. I find this quite believable, as one of the aims for the project was to be able to monitor a 20MHz wi-fi channel at 5.8GHz, so that the 802.11 protocol could be implemented entirely in software.
The only reason the RTL devices are limited to smaller bandwidths seems to be a limitation in the performance of the RTL2832 chip itself - something not entirely surprising to anyone familiar with the performance of other Realtek products ;-)
Cheers, Adam.