I have just added a script that backfills 1 group completely. It gets 20k headers per thread per loop until it has completed the group or reached your backfill days. The group is selected by the oldest post, so the furthest backfilled group gets completed first. This gets the easy groups out of the way first.
In my tests, I backfilled a.b.inner-sanctum completely in ~90 min, ~48 mil headers, using 28 threads. I was not running releases at the same time and it took ~5 hours to create all of the releases from the group. Pay attention, this can overwhelm your db, but is better that backfilling 20k per thread on different groups.
You can run the script from:
python -OO misc/update_scripts/threaded_scripts/backfill_safe_threaded.py
or select Safe Backfill in tmux.