Absolutely. IMO the HM TC adds are the hardest threat challenge in the game right now, unless there's something in NiM TFB/S&V I don't know about.
I'm going to agree with this, and I've done the Nightmare content. Kephess is pretty annoying to tank because of the circle thing, but it's just a question of positioning. Nothing else is of particular interest. The HM TC adds are legitimately hard and require a significant amount of attention. Saber Reflect makes it almost trivial though, so raid groups who roll double-Guardians can just swap off and easy-mode it, but everyone else has to work.
Regarding the other suggestions, I want to avoid things that require an instance, just because I want a) the time commitment to be reasonable, and b) the setup to be fairly low. I don't want it to require tedious clearing in order to get to the object lessons. Though, solo tanking NiM Jarg and Sorno is an awesome suggestion. :-) I did this once a few months ago with four DPS who were (at the time) parsing over 2.9, one of which who was a Focus sentinel that got a 3.3 on the fight. It was one of the hardest bits of tanking I've ever done, strictly on a planning and threat tabulation metric. Really fun. But also requires an instance, so…
All of these tanks are familiar with how to tank KP, so I'm not sure that any of the bosses in there would make for a good object lesson without doing something cheesy, like taking off gear or AFKing the healers.
I do somewhat like the HM FP suggestion, though it would have to be one of the newer ones since the 2.0 HM FPs are absolutely terrible for tanks of all shapes and sizes. The tanking mechanics on those bosses are so stupid (not challenging, just stupid), so it's hard to glean information. The other thing that's tricky about HMs is a) people have run them a bazillion times, and b) it's not entirely like what you see in a raid, where you have a second tank to coordinate with. Not sure how to resolve that issue, or if there is even a way, but I thought I'd put it out there.