Well, with this setup you will not have any improvement with flow offloading! No routing/nat involved...
I suggest you haven´t tested it this way with factory fw? I don´t think you have iperf running on tp link´s fw.
Your test environment should be:
computer (should have a bit horse power) <> [wan] c7v2 [lan] <> computer (should have a bit horse power)
Iperf is a bit heavy on cpu! So if you want to benchmark the routing/nat throughput of a low powered router device, you should not run any cpu heavy stuff on this device.
My WDR3600 does reach about 190 MBit/s over wifi with flow offloading:
notebook [wifi: intel 8260] <> [5g wifi] wdr3600 [wan] <> my server
About 920 MBit/s over lan with flow offloading:
notebook [realtek XXXX] <> [lan] wdr3600 [wan] <> my server