no idea on the performance, all I know is I can get it working, but func_detail is easier.
it seems like the kind of thing you'd only need if a) you were nearing the brush limit or b) you wanted to give it a unique skin, or c) if it was repeated everywhere
I imagine it'd cut down the map size because of fewer baked lightmaps and there'd be less in the texture memory so performance should overall be greater than using brushes, but only noticably if you were doing really detailed objects
Handy tip: if the model is too complex, you can turn it all to func_detail and create a simplified collision mesh using nodraw