How "extreme" is this?

I like most of the movies you mentioned and enjoyed this as well. It's not as gory as most of those but it looks very realistic. There is a lot of human imprisonment, torture and "real" murders recorded from the killer's handheld camera. The rest is done documentary style and follows the police investigation.

Of the films you mentioned I thought this was most familiar with Martyrs but in a totally "this could actually happen" way.

it depends on your level of empathy.

the film is not very graphic, so out of your list it is closest to funny games, since funny games is terrifying, but basically shows nothing, BUT funny games is not a horror film. by no means.
TPK on the other hand is 100% pure horror film, but its is not the graphic violence, that makes it so disturbing, it is the cruelty of the described events and how realistic it is made. usually i am rather doubtful when it comes to "found footage" films, but the mix of found footage, fake tv coverage, fake cell phone footage, fake 911 calls in front of a black screen ...etc makes it damn convincing and that combination makes it so terrifying.

I'm pretty late, but I also enjoyed the films to listed and I say don't waste your time. I found TPT incredibly boring. An hour in, I felt like I'd already watched a two hour movie. Plus the violence and "disturbing" scenes were pretty dull.

As far as being disturbing, this movie is on the same level as "Funny Games." Funny Games had fewer victims, but the torture in Funny Games is longer and more drawn out. In Poughkeepsie Tapes, the torture is shown in little bits, cutting back and forth between police talk and the actual serial killer