Has any one try to use pecans instead of chestnuts? How about any other nuts ?

I'd add a smallish peeled russet, cut into tiny bits, to replace the chestnuts. That will give the soup a bit more body - and russets always make soup taste great. ;o)

Agree with AJ... nuts don't really work as a replacemebt for chestnuts.
In the past, I have replaced them with squash or sweet potato where other ingredients gave body, spuce and crunch.
Here, those vegetables might be nice in flavor but not much good in adding texture.
I might omit the chestnuts altogether and garnish the soup with croutons, noodles or parmesan crisps.

I don't think I'd make the substitution. The chestnuts are starchy, with a milder flavor. I fear the pecans would overwhelm the other flavors here, and that their texture could be somewhat gritty. I also don't think the flavors of pecans + cauliflower work that well together. Of course, I could be wrong! It's all a matter of preference. I'm interested in the thoughts of others about this. ;o)