Loosing a few lumens per watt to a single, larger, HID bulb shouldn't be your primary concern. Everyone looses much more light energy to the actual distance the light has to travel (from source to plant). That's not just my idea - it's the law! It's the "Inverse Square Law of the Propogation of Light". It's Physics, Google it! Plainly stated, it dictates that each time the "Light Distance" is doubled, it's energy is reduced to 1/4. In other words - if the distance is reduced by half, you get 4 times the energy to your plants! To demonstrate what this means, let's assume the OP's 2'x5' space, let's also assume that the lights are mounted across the 2' dimension (perpendicular to the 5' sides).
Think about how far the light has to travel from a single source to reach the outside edge of your grow (in our case that's about 30").
Now think about 3 smaller lights that are truely equal spaced (for even output), that's one in the middle and one about 10" from each end. With this setup, the farthest any light has to travel is about 10".
So, what have we got?
Single big light = 30"
3 smaller lights = 10"
I'm no mathimatician, but I would say that the multiple smaller lights would retain at least 4 times as much of thier energy (probably 6 times as much), as the single big light!
So, it's not so much about which light you start with; as it is how far that light has to travel!
Multiple, smaller, lights are better for a number of reasons! But "The Inverse Square Law" is probably the one that is overlooked the most. Half the distance - 4 times the power -YEAH!
This example goes to illustrate effciency, economy and "even light coverage" for the whole grow. The fact is, "that one plant" (right under the big light) will recieve more intense light energy (at the peril of the majority of the grow).
