I will be having a 5mm downgraft along with maxilla and mandibular advancement and was wondering how stable downgrafts are with the current techniques surgeon use. Most reports I can find on this topic are pretty outdated or perhaps there hasn't been any significant improvement related to this procedure.
The least stable movement there is. But if you need it, you need it. It helps if you don't grind your teeth (you may be at night, which you're not aware of). Different surgeons claim different graft materials are better: porous HA blocks, cadaver bone, autologous bone, demineralised bone, the concoction that A&G use, etc..
But you are right, there are few studies, some contradictory. The plates themselves provide most of the stability, AFAIK. Again, some surgeons use big plates, some surgeons use mini-plates.