Breaking the wall of sound! un0rick can sample at 64Msps but ... one can offset the start of an acquisition by half a cycle. In practice, it means one can do two acquisitions, with one delayed by half a cycle, and interleave the two series. In practice: getting 128Msps acquisition speed!
In picture, it seems quite nice. The signal is loud and clear!

And notebook at https://github.com/kelu124/echomods/blob/master/matty/20180814a/20180814a-Server.ipynb . Some improvement on the lib too, not perfect yet but someday better!