You're already off to a bad start by saying anything negative. I don't know if you quoted yourself letter for letter, but adding "reeeeally" to 'bad' is just asking to get shut down before you can even make your suggestion.
Giving tips to players has everything to do with people skills and timing. Running dungeons with a PUG won't provide many windows. The thing about dungeons is the only time a window really opens up to help a player is if it is going horribly wrong, in which case you get this large window to finally explain things. Otherwise you risk spending more time in there than is necessary just to coach up a player.
You also have to choose your battles. Do you really want to burn up more dungeon time trying to get a SAM to nail his/her positionals or get them to use True North? Cases like these, it is a more efficient use of everyone's time to just suck it up and go instead of risking a "you don't pay my sub" debacle. However, if you have an ice mage, tank not using CDs, healers not healing and other scenarios that are literally destroying the run, then something needs to be said, and I have personally stopped a dungeon run to get everyone on the same page before continuing.
Sure. You just have to make them respect you enough to value your criticism. You also have to get the other members respect and patience to value the time you put into correcting a player's playstyle. That is far easier to write than do with strangers you might not ever see again.Has anyone mastered a tactful way to tell someone that they're severely under-performing when you're going through a dungeon, and there's not a whole bunch of downtime to write a whole essay on how to properly play a certain job?
For the most part, in dungeons it isn't worth the time, effort, and added frustration. Players who want to improve will do it on their own. They'll seek out guides, read and understand their tooltips, spend hours on training dummies, and become open to constructive criticism. You also need to keep in mind that not all players have access or even want to use third party tools to measure their performance, nor are they supposed to be using them in the first place. So if your critique isn't coming from anything other than your observations on the battlefield, I wouldn't say anything.