The expanding inserts are usually pretty flimsy, you can get a better result by substituting an expansion anchor from your local hardware store. I had good results with a solid rubber 'Rawlplug' anchor. It has a brass insert and comes in 5/16 th" thread which is the imperial equivalent of the 8mm bolt usually supplied.
I had bar end mirrors for years, I liked the look but broke heaps of them as they hang out there pretty exposed. They were the same style as the emgo or napolean one on Rob's links.