I am late to this thread, but I have first hand accounts of some info.
I used the old Happy Trail panniers and was extremely happy with them. They took quite a number of drops and one medium speed crash in the desert. Yes, the panniers did go out of shape, but a bit of banging fixes that. The important thing was they stayed on the bike and protected my legs, so they acted as crash panniers, a very important point imho. Since I have been back home, I bought the new quick release HT system and they are frigging useless, they fell off the bike as soon as they touched the ground, both times at medium speed in a desert over here. The metal bent and twisted rather than just pushed in, so they were now not water proof. The worst part was, I was trapped under the bike because the panniers fell off.
I will still recommend the HT system, but get the old, screw on type of system, not quick release. In fact I never used to take the things off anyway, only once or twice when I could not get through the hotels front doors did I have to take one off, the rest of the 11 months they stayed on the bike anyway and I just unloaded what I needed. That could have been because they were a pain to get off, but still, that is the way it was.
Tyres, I ran a TKC80 pretty much all the way from Prudhoe bay to Ushuaia, except for a small stint with an MT60, this tyre will give great grip through the mountain twisties of Colombia as well as the rocks and sand of the desert.
For the back, I tried a number of tyres during my run south, but in the end scored an Anakee 2 and if I had my time again, I would run that all the way. If you are doing a lot of USA slab runs, then a harder wearing ME880 would be my choice.