Been a passenger at 183Mph (approx 192kph), and the car felt stable enough, the wipers didn't like it though That was in MadRod's which is running coil overs, etc...

Don't know what would have happened if a gust of cross wind came out of nowhere though

Not many people have been up to that speed in a Sierra though. Only people I know who have are :

MadRod
MADevelopments (mark)
MadAde

And just to show I am not biased , Martin Reyland's escort was probably capable of those speeds.

There probably are others, but I don't know them.