Ultimately, managed to test different combinations of "Packet steering", "Software flow offloading" and irqbalance enabled individually, as well their combinations.
2.4GHz download: "Packet steering" produces the highest speed (74Mbps), which is at par with stock firmware download speed (78Mbps). "Software flow offloading" is second best (68Mbps), almost all other options are below 50Mbps.
2.4GHz upload: all settings produce more than 90Mbps speed, e.g. irqbalance - 95Mbps, "Packet steering" - 93Mbps, "Software flow offloading" - 91Mbps". The highest speed is for the combo of irqbalance and "Packet steering" - 98Mbps.
Based on these results from 2.4GHz radio, I am leaning to keep "Packet steering" enabled.
5GHz download: "Packet steering" produces the highest speed (438Mbps), still below the stock firmware speed (605Mbps). The combo of irqbalance and "Packet steering" is second best (423Mbps).
5GHz upload: somewhat mixed bag. irqbalance + "Software flow offloading" + "Packet steering" win with 534Mbps speed. "Packet steeering" + "Software flow offloading" second best with 485Mbps. irqbalance alone produces 477Mpbs, and "Packet steering" alone - 467Mbps. Stock firmware is at 464Mbps.
For 5GHz, "Packet steering" again seems to be the optimal option (best for download, but only OK for upload).
All-in-all, having "Packet steering" enabled seems to raise OpenWrt Wi-Fi performance up to the level of firmware Wi-Fi performance for both radios, with 5GHz download being the only exception, lagging by some 166Mpbs the firmware download speed.
@pythonic Thanks a lot for pointing me to these settings!
I checked with cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor, the output was performance.