Мой косяк, аднака...
Пересобрал с обоих сторон iperf с поддержкой pthreads.
При 'policy 10000000 65535' на порту пустил 'iperf -c ... -P 20 -t 120'
Получил на сервере:
[  7]  0.0-120.2 sec  7.20 MBytes    502 Kbits/sec
[  8]  0.0-120.5 sec  7.34 MBytes    511 Kbits/sec
[ 12]  0.0-120.5 sec  7.75 MBytes    540 Kbits/sec
[ 13]  0.0-120.5 sec  7.52 MBytes    524 Kbits/sec
[  6]  0.0-121.4 sec  7.16 MBytes    495 Kbits/sec
[  9]  0.0-121.4 sec  8.21 MBytes    567 Kbits/sec
[ 10]  0.0-121.4 sec  7.55 MBytes    522 Kbits/sec
[ 11]  0.0-121.4 sec  7.08 MBytes    489 Kbits/sec
[ 16]  0.0-123.1 sec  7.94 MBytes    541 Kbits/sec
[ 15]  0.0-123.5 sec  7.96 MBytes    541 Kbits/sec
[ 18]  0.0-123.5 sec  7.27 MBytes    494 Kbits/sec
[ 20]  0.0-123.5 sec  7.34 MBytes    499 Kbits/sec
[ 21]  0.0-123.5 sec  7.47 MBytes    507 Kbits/sec
[ 19]  0.0-123.9 sec  8.02 MBytes    543 Kbits/sec
[ 14]  0.0-124.3 sec  7.24 MBytes    489 Kbits/sec
[ 17]  0.0-124.7 sec  7.38 MBytes    496 Kbits/sec
[SUM]  0.0-124.7 sec    120 MBytes  8.10 Mbits/sec
Что уже приемлимо.