Look at the Kernel Log or use the following command in ssh terminal...
dmesg | grep 'firmware ver'
Output from what OpenWRT package loads...
root@OpenWrt:~# dmesg | grep 'firmware ver'
[ 15.395023] ath10k_pci 0000:00:00.0: firmware ver 10.4b-ct-9980-fW-13-10af6a005 api 5 features mfp,peer-flow-ctrl,txstatus-noack,wmi-10.x-CT,ratemask-CT,regdump-CT,txrate-CT,flush-all-CT,pingpong-CT,ch-regs-CT,nop-CT,set-special-CT,tx-rc-CT,cust-stats-CT,txrate2-CT,beacon-cb-CT,wmi-block-ack-CT,wmi-bcn-rc-CT crc32 4a0fa880
Output from latest Beta non-commercial...
root@OpenWrt:~# dmesg | grep 'firmware ver'
[ 15.627184] ath10k_pci 0000:00:00.0: firmware ver 10.4b-ct-9980-xfW-13-5ae337bb1 api 5 features mfp,peer-flow-ctrl,txstatus-noack,wmi-10.x-CT,rxswcrypt-CT,ratemask-CT,regdump-CT,txrate-CT,flush-all-CT,pingpong-CT,ch-regs-CT,nop-CT,set-special-CT,tx-rc-CT,cust-stats-CT,CT-STA,txrate2-CT,beacon-cb-CT,wmi-block-ack-CT,wmi-bcn-rc-CT crc32 cecd9aea
But, with the speeds you are getting using TP-Link's Stock Firmware I don't think OpenWRT or any ath10k Firmware changes are going to make a difference...
I'd be checking for TX/RX Error and Retry rates on the 5Ghz radio. Might be other devices using the 5Ghz band around you bringing your noise floor up. Common things are Wireless Security type cameras and sensors, Cordless Phones, Radar, etc...
Even if you are right next to the AP with other "stuff" operating in the same band around you. That can cause high Packet Error and Retry rates. Which will naturally slow your usable speeds down. But, since any Wifi radio tries to wait for "clear air" before they transmit. That wait timing can slow you down too.
Even at the worst using the ath10k-ct Driver/Module and ct Firmware that OpenWrt master is using. I still get 400Mb/s Up from Client and 250Mb/s Down from AP using iperf3 and UDP transfer tests. The Beta CT Firmware seemed to make the results more consistent. Going to the base kernel ath10k Driver/Module opened my Down from AP speeds more since the AP radio actually used 80Mhz width with it.