as JonnyM already said the power amplifier(s) cannot deliver more output power than what they have been designed to do even though the radio chip and driver software are providing and have been set to do more power.
When you look in the specifications of power amplifiers there is an issue with them starting to amplify less linearly when pushed to their limits, or in other words they start to introduce distortion. The sophisticated modulation schemes used in 802.11 (the way the radio signal is given its data content) require very linear amplification (the output being a precise multiple of the input). The effective data throughput may actually suffer as the linearity decay destroys more than the bit of extra power brings. For best performance it is wise to set the system a few dBs under the limit the power amplifier has been specified for (to account for calibration inaccuracies and spread between systems). Non linear amplification may also produce spurious signals.
There are only a few manufacturers of these amplifier ICs which require special production processes and you may have a look with a magnifying glass which actual PA chip has been designed in and check out its specs. Could not read the type numbers from the FCC photographs but they appear to be ICs AU6 and AU7 under the metal shield where antenna connectors AJ1 and AJ2 are located.
This may be all true, but I have seen real world signal strength increases by raising the radios to their limits.
With 17dBm (defaults), the range of the router barely reaches the 2nd story of the building, and doesn't get very far outside the building.
With 24dBm, it effectively reaches the 2nd story without being on the verge of losing connection all the time. It also allows me to travel further outside before I lose connection.
So, these radios seem to handle the extra power just fine.
What's disappointing is just how short the 5GHz range is. Even with 24dBm, it doesn't get much faster than 54Mbps on the 2nd story, no matter that it isn't very far (10 meters?).
My old WRT54-GL seemed to do much better with 251mW set. It would easily reach the furthest places on the property. Of course, the WNDR3700 can supposedly reach that, but I haven't tested it with a meter yet.
I may have to find the optimal location for the 3700. Frankly, the 4th gen Airport Extreme I have has better 5GHz range and throughput. I'm guessing the antenna configuration (and location) is responsible.
The 5th gen Airport supposedly does 400mW in most situations. So I don't thinks it's unreasonable to put out power that high (the FCC allowed it).