To bring an end to this confusion:
Yes, FSO does not use simplified collision models.
No, that is not in and of itself a performance risk, because we are using something called BSP trees to subdivide the geometry. This means that, when a collision check is performed, the engine uses a succession of abstractions starting from the models' bounding box, to a subobject's bounding box within that, to a specific polygon's bounding box until it gets to the specific polygon.
Now, this tactic has worked perfectly well for retail-era models, it's only recently that it has shown its age, given the level of detail present in modern models. It is possible to do collision models (seriously, the framework is already there in the models, and in particular the $nocollide_this_only submodel property), but it's not something that is in wide use, or really tested that much.