My experience is, frankly, limited. However, flying over Scottish mountains of about 4,000ft, I've found that to clear most significant "funny wind effects", 6,500ft does it.
What I don't know is whether the main player is that I was about 2,500ft above the peaks, or that I was at about 160% of the height of the peaks.
The other player, at least for me because I don't get to play with multi-engined aircraft very often, is that I'd plan it on the basis of a consistent series of viable glide diversions which are, at the very least, survivable.
G