Odd that sequential write performance has dropped so substantially, when everything else including random writes shows a small boost. I'd always thought of sequential writes as being the "easy" bit to do quickly, too - getting much harder when you get lots of small fragmented writes instead.
There seem to be some other oddities in the lineup, though. The SM1625 is listed as coming in sizes up to 800Gb - with much faster random writes but slower random reads than the smaller units of the same model. Slower sequential reads, too (700 instead of 900 Mb/sec) - but sequential writes at 450, right in between the 100Gb unit's 300 and the 200Gb one's 600.
I haven't seen a teardown yet of how many Flash modules, what size and what the channel arrangement is. If the SM1623 had six channels to the SM1625's five, that would explain the random read and write improvement, but why the drop in sequential write? Is it doing more intensive checksumming or readback validation to wring better life out of the smaller MLC cells?