Just to throw in my anecdote: when touring a local datacenter some years ago one of the principals pointed out equipment cages they claimed were rented by the NSA. The facility wasn't the local carrier hotel, but did have a large number fiber loops into it, enough that peering with virtually everyone was trivial. It would be a reasonable place to duplicate and sift through traffic that goes out over a number of pacific undersea cables. It's also entirely possible the principal was telling a tall tale.
In any case I think given all that's been disclosed over the last 10 years it's more reasonable to assume over-collection than under-collection.
Undersea cables go to other nations, and thus aren't domestic.
The idea that NSA has 'dozens, if not hundreds' of devices 'in every major US city,' regardless of utility for foreign intelligence, is just rampant paranoia, not much behind claiming that the lizard people are eating our brains while we sleep.
In any case I think given all that's been disclosed over the last 10 years it's more reasonable to assume over-collection than under-collection.