In the last Epik rev, outdoor FR graphs were shown on the product pages. They really don't tell you much other than how the sub behaves at moderate volumes with no reflections. In room measurements from users from the Empire are room dependent, but the one's I've seen show that the sub is capable into the midteens, as advertised.
Personally, I think the reason the Empire's graph isn't up is that it exhibits a hump in the midbass and rolloff below 30hz, which is fine for a sealed design, but might make some hesitate in buying if they don't think they'll acheive the desired extention in their room.