We have escalated this as far as we can with vmware Engineering.
They have come back last week and told us it is "expected behavior" and there is nothing wrong with the product. There are no fixes - It is simply our deployment and use case (to expect even moderate sequential write performance) that is wrong.
Yep, that's right. An enormous drop in performance with checksumming is not a bug at VMWare, it is expected behavior!
VSAN can't even keep up with the spinning disk SAN it was purchased to replace, and all we have gotten is what appears to be a cover-up from support... VMWare has lost our trust.
We are talking to an alternative HCI vendor now to see how quickly we can move across.