Not persé but adding stripes is counter productive, especially on all-flash. Normally, in a Mirror, data needs to be fetched from only two capacity devices. One on say, node 3 disk 2 and the other mirror copy lies on node 6 disk 4 (just an example). So the data needs only to be written and read to and from 2 devices.
If you start striping, say a stripe-width of 2, that means that each mirror copy is now actually made up of 2 parts so 4 parts in total. Now, reads and writes are done from 4 devices. This adds latency. The higher the stripe-width, the higher the latency becomes.
Striping has only 1 use-case and that is on slow rotational drives where sequential performance (streaming etc.) is important and latency is not.
In other words, you just made the latency worse for yourself ;-)
Also, from a pure data point of view, it sounds like you have absolutely no use case for a stretched cluster if both sites keep their data to themselves anyway. Why not have 2 regular clusters. Much simpler, no messing about with a Witness Appliance etc.
In case of an emergency, you don't have the time and means of moving all the data to the other side so quickly anyways because you never replicated a thing to the other site.
Or is it just a small number of VM's which are, policy-wise, not stretched but all other VM's are mirrored in both sites?
My advice: stripe-width of 1 as Flash is so fast, you gain nothing with striping. It just slows everything down as data needs to be written to and fetched from more devices than it needs to.