I would be as happy as you are, if I wasn't painfully aware of the 100s of millions of wifi APs shipped in the last 4 years that behave as per your "before" result. And I think it's still possible to do a great deal better on the _be test, but honestly I don't know enough about batman's encapsulation to tell.

could you slam the associated *.flent.gz files up somewhere?

For this test:

flent -H the_server_ip --step-size=.04 --socket-stats --te=upload_streams=16 tcp_nup

If you can get a packet cap of the "after" result (from the server) I'd love to see that. (tcpdump -i whatever_the_device_is -s 140 -n -w tcp_nup.cap) (note if you run the capture on the ap it will slow it down by a lot and heisenbug the result, better to be on the ultimate target)