That is correct: TSA requires batteries less than 27,000mah. My computer uses 170W at full draw.
Since MBP's charger is 90W, I find it puzzling why this is even being discussed. The bottom line is that external batteries are very much viable for a MBP and completely within the limits imposed by flight regulations. So yes, external batteries are a simple and affordable answer to the lack of user-replaceable battery on the MBP, not to mention that they are also more flexible, can be charged separately from the laptop and do not require you to power down the machine to be replaced.