Ok
On WDR4300 OC to 730MHZ
Up Down limit on eth0.2 interface set to 800000kbit/s = 800Mbits
jperf cake layer_cake = 571 Mbits/sec
jperf fq_codel simple.qos = 663 Mbits/sec
Throughput definitely increased but not line rate.
I guess lower cpu utilization allows better SQM
So to answer the question:
No you will not get line rate with SQM even with fast-path but performance will be better than without fast-path
How much performance you get with SQM still depends on the CPU.