Reviving the dead. I stumbled on some history of this topic here: https://patchwork.kernel.org/project/linux-wireless/patch/1437622380-4154-5-git-send-email-wens@csie.org/#14634341
The US FCC increased the maximum conducted transmission power for the U-NII-1 (5150 ~ 5250 MHz; channels 36 ~ 48) band to 30 dBm or 1 W for master devices and 24 dBm or 250 mW for mobile/portable devices in 2014. See FCC KDB 905462 D06 and https://transition.fcc.gov/bureaus/oet/ea/presentations/files/oct14/51-New-Rules-for-UNII-Bands,-Oct-2014-TN.pdf.
The wireless-regdb has no mechanism to distinguish between AP and client mode, therefore it restricted US transmit power to 250 mW for both AP's and clients. The wireless-regdb maintainers further decided to conservatively round 250 mW down to 23 dBm for the U-NII-1 band only, which is not consistent with implementation by wireless-regdb of the same 250 mW limit for US U-NII-2 as 24 dBm.
It seems necessarily up to the firmware, not wireless-regdb, to allow US U-NII-1 transmit power up to 30 dBm depending on whether the firmware is operating the device as an AP and not a client. There are numerous examples of OEM firmware legally allowing US U-NII-1 transmit power up to 30 dBm in AP mode. OpenWrt firmware does not. Instead, OpenWrt reduces allowed US U-NII-1 AP transmit power by a whopping 79% (30 dbm to 23 dBm) across the board, because it defaults to the wireless-regdb. If OpenWrt is ever to support US U-NII-1 AP transmit power of 30 dBm, it seems this must be addressed in OpenWrt. It's not clear how this could reasonably be "fixed" in wireless-regdb without restructuring wireless-regdb to distinguish between operating modes, in which case changes to firmware, including OpenWrt, would still be required to make use of it.
It would also be nice if the wireless-regdb could consistently implement a US 250 mW limit as 24 dBm. Slapping a 17% reduction (23 dBm vs. 24 dBm for a 250 mW limit) on U-NII-1 only is just strange, but that doesn't seem appropriate to address in OpenWrt when this more minor issue is with the wireless-regdb.