I require a simple clarification. If you perform a K-turn and then boost as your action, do you boost after turning around or do your continue as though on your original trajectory and then turn around afterwards? My interpretation of the rules suggests that since the perform-action step occurs after the movement step, you are already turned around when you pull off that action. Is this correct?

It depends on what pilot ability/dial you have.

All normal pilots= you would not be able to boost after the K turn as you are stressed from it and cannot perform actions or free actions

Tycho, Tie defender = After the K Turn you would boost so you would boost in the direction you would be facing after you K turned.

A K-turn is a red maneuver and causes your pilot to receive a stress token. You may not perform actions whilst you have a stress token. If you were able to K-turn and boost, the boost would be after your maneuver. For example... a TIE defender can perform a white K-turn and if it was equipped with Engine Upgrade, you would then be able to boost after your movement because the defender's K-turn does not induce stress.

Yes, I'm aware of the stress issue, but there are a few ways for that problem to go away. I was indeed thinking of a TIE/D with an Engine Upgrade, but I am aware of some other ships that can remove stress from allied ships. Whether that would occur in time for the recently stressed ship to use its action is another matter, and may be dependant on the particular combo, I haven't looked into those in detail yet.

Adrenaline Rush and Captain Yorr would both allow other ships to K-turn without getting stressed.

Adrenaline Rush let's you treat a red maneuver as a white maneuver, so you'd be able to perform an action.

Captain Yorr prevents the friendly ship from receiving a stress token, and so they can continue to perform actions.

And there's Advanced Sensors. Since you perform the action before you reveal your maneuver dial, it won't matter if you reveal a red maneuver.

So Tie-Phantoms with Advanced sensors could boost or barrel roll or cloak, and then do a K-turn. A Phantom could actually decloak, do the decloak barrel roll 2, Cloak, decloak again and roll another 2 to the side, and then perform a K-turn.

This is a special case because spending a cloak token to roll or boost 2 doesn't count as an action. Stygium Particle Accelerator wouldn't let you gain 2 evade tokens, because it gives you an evade as a free action, and you can't perform the same action more than once a turn, even if it is free.

So Tie-Phantoms with Advanced sensors could boost or barrel roll or cloak, and then do a K-turn. A Phantom could actually decloak, do the decloak barrel roll 2, Cloak, decloak again and roll another 2 to the side, and then perform a K-turn.

Yes...move first than action as long as you are not stressed, or if you have advanced sensors.

To more directly answer your question, ignoring the Advanced Sensors scenario, you complete the maneuver you have selected, then have the option to take an action. So before you have the option to boost you've already performed the K-turn and are facing the other direction.