Nigezu,
How many models are you putting into a single area? The highest number of polys in that collection is around 600 (the Artillery Crate). You may be running into the problem others had with extra geometry in certain parts of the segments (it was discussed with TGC, but they found there was nothing they could do about it at this time).
Try a new level with only 1 prefab (such as the large warehouse), fill it with some of the pack's entities (I filled a prefab with about a dozen of each, as well as 5-6 enemies, and weapons without loss of framerate), and try it out.
Let me know what the results are.
-Keith
"Some people are only alive because it's illegal to kill them".