Please note that there are many variables here. The legal limit set by the FCC is to never transmit OVER 30 dbm. To accomplish that, the average power setting will be lower since the peaks must remain under 30 dbm. The dbm being reported in DD-WRT firmware is NOT the peak power, but the average power setting. It also doesn't include gain from antennae, etc. So 20 dbm power setting gets close to peak power of 30 dbm. It is similar to comparing audio amplifiers and comparing peak power to RMS/average power.
The FCC report logs the peak power measured out of the antennae, not the internal chipset setting. As some have found, the performance is superior to competitors in real-world testing. Of course, everyones' mileage may vary, but please also know that since Buffalo HP routers are flirting with the limits of FCC, certain channels have to be detuned additionally to keep peaks down.
Also, spurious emissions outside the band of 2400-2483 have different limits, so lower and higher channels (e.g. CH 1 and 11) have additional requirements for lower power to keep their spurious emissions below the cap.
Center channels will give optimized performance. Additionally, the nature of Wi-Fi is that 20 MHz will transmit at higher power as well, so to accomplish max power, 20 MHz channels and center channels should be selected.
-Brian