My personal limits a roughly as follows:
Up to 10 knots no real problems including flying up the valleys.
10 to 15 knots stay 50% above the top of the peaks and ridges, I.E. ridge/hill top 4000 msl I fly at 6000
15 knots plus I stay out of the hills
I'm a bit of a chicken coz I hate getting my ass flung about over the Grampians. Not entirely sure why but my aircraft seems to get every bit of turbulence and jostling that's going, I was told once it was something to do with the low wing loading, my old piper never seemed to bounce around as much, then again it ain't gonna matter what spam can your in if you hit heavy down drafts/rotors your gonna get slammed