I didn’t try 2800, but I did run at 2933, 3000 and 3200 with 64 and 2933/3000 with 64. Seemed like a pretty linear improvement at each step. More IF speed = more goodness.
One thing I’ll say from Intel memory OC world, DDR4 re-tries can silently eat into your memory performance when you’ve pushed it to the ragged edge. Just before you start getting reported errors, you are getting silent re-tries from CRC errors at the DDR4 PHY level.
This is often what’s going on when you start seeing performance degradation at higher speeds without reported errors (assuming your net latency is reduced by the higher speed and not just trading clock rate for cycles on primary/secondary timing).