I understand it this way. Default BI's character model, with all the equipment it has, was made for the computing power of processors from almost 10 years ago when this game was published. The DLC's models could be made for the power of today's and future gaming rigs, so one might assume they are more detailed. The difference may also come from the way of how a particular addon was made - if it has higher resolutions of bitmaps and whether the character model has sufficient resolution LOD models for itself and its equipment. Nonetheless, the game engine (as I understand it) tries to keep dynamically a playable FPS all the time, using LOD switching. So, generally speaking, if there is a drastic drop of FPS without noticeable degradation of model quality (especially those in a distance) it may point out to lack of sufficient LOD models. I don't know how you did your tests, but I'd have done it in VR to get coherent results.