I think the 16 inch pro already uses the maximum capacity of a battery allowed on airplanes by FAA rules.
If you had an 18 inch and could not get a bigger battery than the 16 already has, that means it would not last as long. Though that may not be a huge issue as long as it can get 7+ real world hours of productivity.
The limit is 160 Whr, you just need to let the carrier know during check in. The limit is set by IATA, not FAA.