I flew from Calgary International to Cranbrook and back last week - took the shortcut and went direct over the rocks at between 8,500 and 10,000 feet. It was a beautiful sunny day, just a delightful flight in both directions.
Even then, though, I was quite surprised at the amount of orographic turbulence that I encountered. At various times, the aircraft was going up (or down) at 1,000 FPM just due to airflow over the mountains.
I felt comfortable doing the trip - I was flying an empty Twin Otter, and thus had a pretty comfortable reserve of both horsepower and rate of climb - but I don't think I would ever try this shortcut with a less powerful piston aircraft.