I have not yet seen any cheaters yet either (about 100-150h game play).
I have seen some assaults move fast, but they were using blitz and I did not notice.
For the wall hacks: There is sometimes a delay between your death and the turning towards the killer (depending on the side you were facing and where the killer was at that moment). In this delay the killer could have moved himself and when you see it, it might look like he shot through a wall (or whatever object he shouldn't be a able to shoot through). I have seen a lot of people calling others wall hackers because of this.
A kill cam would most likely prevent wall hacks though (or at least minimize it).