Back on IA when the sentinel's OD bugged and didn't do damage, there might have been someone to break 400, but in normal gameplay, I'd say that its simply not possible.
Back on IA, I once clipped through the map on Zenith Cauldron. Curious to see how fast I could go, I didn't suicide for about four minutes. My velocity topped out at 730 m/s.
ive fallen through the bottom of the map before as well and reached terminal velocity for a legionairre i think it was 758 m/s
this was ages ago through.