Kukuk the enemy know if you have the crosshair on them. Don´t aim by the sight at a crouched enemy and he will pop up much sooner.
As for loot I found the best in the main story generally except for the arenas. But this much depend on what level you do them at I suspect. On the main quest you are guaranteed these rare items at least.
Speaking of that I find the 5 % chance for rare drops artifact annoying. It means I have to use it just about always while it doesn´t make me stronger. I did now find an artifact that increased the xp points for killing enemies that I may want to change it for to level up faster :)
True vault mode is quite challenging at the moment. When you get these super badass one level above you really have to find a place where you can exploit the AI turret or not :p