Thanks for the further opinions everyone! For reference I'm using a Parker shavette + Shark blades. Fully aware that this may not work for everyone, but I've been continuing to test purely ATG for the last couple of days:

* Sideburns/jawline: Just as smooth as multiple passes, but no irritation at all. No difference in time needed between shaves either.
* Chin: The parts that get done easily (centre of chin) are great but the corners of my chin need a touch-up pass. Think Ernie1980 is right with the extra passes catching stray hairs, two passes doubles the chances of the hard-to-reach hairs at least getting a trim. With that said, I'm happy to say that a full ATG pass then a touch-up gives no irritation if I stretch my skin properly, while there's a bit with two passes.
* Upper lip: Tried this once more, but won't be doing it again. Back to two passes for sure!