Translated the linearity of your PA (nevermind LNA for RX) is unlikely to support even 20dBm, and the higher rate modulations for even 802.11a/g (nevermind any MIMO/SDMA workings) are EVM limited, not received power limited
It's peak-to-average power ratio (PAPR) or Crest factor [1].
It's not a really a big poblem for those who know, the limiting PA can be replaced or bypass with with more accommodating ones that can transmit higher power with better linearity and power envelope tracking.
Translated the linearity of your PA (nevermind LNA for RX) is unlikely to support even 20dBm, and the higher rate modulations for even 802.11a/g (nevermind any MIMO/SDMA workings) are EVM limited, not received power limited