I've been using 5 MB/sec for my experimental transmissions, but I have been hearing from Stefan that European hams have had success with 2 MHz wide 2 MB/sec DVB-S. So on a whim, I set my ATSC transmitter to transmit 2 MB/sec MPEG2 video. And the resulting video looked just fine to my eyes. It got a little blocky when I panned the camera around, so with high motion subject matter, it's not enough. But it's clearly enough for the average QSO.
I still think the receivers should be configured for as much bandwidth as the bandplans will allow (6 MHz on 23cm and 4 MHz on 70cm). If for no other reason than it's conceivable that HD pictures could be sent with H.264 inside 6 MB/sec, and someone might want to try that (despite the fact that it won't be compatible with ordinary TVs).