I have a rooftop tent and side awning on my Ti rails. It's probably 120kg (260lbs) all up, then we have 2 people sleeping in there, so peaks around 300-350kg (660-770lbs) overnight.
Nothing has broken yet but I'm also not precious about dents and damage.
If anything, the feet of the factory rails are probably the first issue, as they are plastic. The rails are just some aluminium extrusions held between the plastic end legs.
I made my own cross bars, but they are no different to any off the shelf ones you'd buy. I do notice that the extrusion rail now squeaks in its foot when driving over any bumps. The extra weight up high is very noticeable in body roll around corners and has dropped my fuel economy by around 1 L/100kms