With the scope set to 10V/div, 80V full scale (313mV/count for 8-bit raw data), plot showing the DSO collecting 10.0V from my Fluke standard for about 20 minutes. The DSO had been running about 2 hours before I collected this data.
I used 3X over the 2400Hz (40th harmonic) with 12-bits, or about 1.8MHz, and set it to 2. I have not tried to push it out another bit yet. 10MHz, for a half second seems very doable.
The transfer rates are no problem with it. I am collecting about about a half second of data, or 30 cycles. This is the whole delay, waiting for the next half second.
My plan was to use this to compensate for the drift.
Anyway, looks like I could improve on my initial measurements somewhat.