transmit power achievable depends on data rate (modulation order). Higher the speed, the lower the transmitter power, in certain steps, for any specific product. This is the nature of the OFDM modulation used for all but the lowest speeds. It has to do with the peak-to-average ratio of power, where the ratio increases as the data rate increases (bits per Hz per second). As the ratio increases, the peak power has to reduce to avoid transmitted signal degradation ("rho").
The rule of thumb in OFDM is to reduce the transmitter power by about 5-6dB at the highest data rates where the ratio is the greatest. Good WiFi gear does this automatically in the firmware. The FCC doesn't care about distorted transmissions, but the WiFi alliance compliance requires a given rho (quality). Not well enforced and there's a lot of crummy firmware out there.
Simplified:
Crank up an audio amplifier too much and you hear clipping and distortion. The solution is a better amplifier with more "headroom". Consumer WiFi doesn't do so because this headroom costs a great deal and WiFi is a very competitive market.
These things apply, no matter what a user interface like DD-WRT says: your input is a request to the firmware, which may well be ignored.
Lastly, there's the imbalanced link issue: The WiFi router may out-power the WiFi client and create a "I hear you but you don't hear me" situation.
