Stems are a pain.
With the high bars people run now it is asking a hell of a lot for any stem to clamp the bars properly without crushing the clamp area. If you use 7075 then the extra hardness (and its a lot) makes it harder for the knurling to bite in, so you need more pressure to get the same kind of grip... having said that, the extra strength means that a 7075 stem is less likely to slip IF you can get the tension just right so that it doesn't crush the bar and isn't too gentle so the bars slip and chew it out before you get that far. I think that 6061 is a "safer" bet for most riders and will get the necessary grip more easily without the rider going too far and crushing the bars.
Tree may do their collet stem in 7075 (if they still make it?)
G.