OK - so I ordered two Optane Modules an we'll see, what happens - I'll inform you about the result.
Here is the requested benchmark with 4M blocksize:
journal-test: (groupid=0, jobs=1): err= 0: pid=30815: Wed Sep 6 08:01:19 2017
write: io=12424MB, bw=105998KB/s, iops=25, runt=120023msec...