MT7922: TX power clamped to 2 dBm (tmac=4) on ASUS GA503RM — 450 Mbps vs 600 on Windows

ASUS ROG Zephyrus G15 GA503RM (AMD 6900HS), MT7922 [14c3:7922], mt7921e.

txpower_sku shows tmac = 4 (≈2 dBm) for every rate while eeprom rows allow 32–40:

                      6m     9m    12m    18m    24m    36m    48m    54m
OFDM (eeprom)   :     39     39     39     39     36     36     36     34
OFDM (tmac)     :      4      4      4      4      4      4      4      4

Result: 50–60% TX retries under load at −41 dBm on a clean channel, TCP plateaus at ~350–470 Mbps. Same laptop on Windows: ~600 Mbps, same AP/channel. iw reports txpower 3.00 dBm (aware of the pending display-fix series; the tmac table + retries suggest it's real here, not cosmetic).

Ruled out — no effect on tmac

• Kernels 7.0.0-28 (Ubuntu) and 7.1.4 mainline
• Firmware 20260224 and 20260605 (latest); June build helped throughput ~390→450, tmac unchanged
• disable_clc=1 with full mt76 stack unloaded first (the fix from Debian #1105211, same chip, different laptop)
• ACPI SAR: DSDT MTDS limits are 8–14.5 dBm; patched them to 22 dBm via initrd table upgrade (verified live in /sys) — tmac still 4
• iw set txpower, set sar_specs, reg re-set (US, 30 dBm allowed), deep-sleep/runtime-pm off, powersave off, BT off, ASPM off

Questions

1. What besides eeprom/CLC/regdomain/ACPI SAR can clamp tmac to 4 on mt7921/mt7922?
2. Any fw_debug/MCU trace I can capture to show where the limit is decided?

Happy to test patches; full txpower_sku, eeprom dump, ACPI tables, pcaps available.
FW: Build Time 20260605203411. (AP quirk note: Xfinity XB8 gateway sends corrupt basic HT-MCS, locally patched mac80211 ignores it — unrelated, HE negotiates fine; Windows hits 600 on this same AP.)

Start with the mt7921e driver path that produces the txpower_sku tmac table, comparing its reported values with the eeprom rows. Reproduce on the ASUS GA503RM using the listed kernel and firmware versions, then investigate the remaining clamp sources and any available fw_debug or MCU trace. Done means identifying where tmac becomes 4 and providing a focused fix or diagnostic evidence.