I have Sidi boots for the street and shoes for the Mountain bike.
I have the Vertigo Corsa for the street and love them. I have read first hand accounts of people crashing in them and with the feet walking away unscathed. I personally think they are the most comfortable I have ever worn, although not cheap by any stretch of the imagination.
Also not cheap are the Dominator 4's I have been wearing for three years on the mountain bike. Same deal, very comfortable, I love them. Great shoes.
With my experience in these two genres, I would say that they probably make a phenomenal dirt boot. If you like them, I would recommend them.