The modem is quite unstable, though. See the other topic: Problem with modem in ZTE MF286R
Regarding 2.4GHz, there is not much to do for ath9k than load calibration from flash data - that's what what drivers both in stock (QSDK) and OpenWrt do anyway.
For testing ensure that both firmwares use the same, and mostly empty channel if possible, and with the same channel width. On 2.4GHz, OpenWrt defaults to 20MHz channels, this might explain the difference, although I haven't really looked into 2.4GHz results.
@chunkeey, I found other device (Asus RT-AC57U) using the same boardData.bin file as this unit in its stock firmware - is it good enough premise to conclude, that custom board data is not needed? The test results also look very similar. OTOH I decoded and compared internal board data from OpenWrt versus stock, and there are substantial differences in binary. Are there any tools to parse them for wave2 chips?