11mm at 5k would be a challenge. Our latest BX4 rig is very compact and lightweight but current minimum (*tested*) fl is 16mm at 4k. The mirror box is 14" wide, and we have yet to do any testing on this rig with Angenieux 16-42 lens hoods removed. This would allow us to get much closer to the mirror, and might give us 16mm at 5K (what is that equivalent too? 14mm?). Certainly a wider and taller mirror box would give you more, but as you've suggested it starts to get out of control pretty quickly. The predecessor, BX3.5 has a 19" mirror box which, while considerably larger, isn't unwieldy. Unfortunately the mirror boxes aren't currently interchangeable. BX4 mark II coming any day now, should be right around 12.5lbs bare.