That's a valid concern if you design the box so that the sockets are bottom-to-bottom on a strip down the middle. But what about the case where the size of your box requires only two rows of lamps. In that case, you could mount the lamps with their bases on the outside. I would think that would avoid the banding problem.
The basis for my question about horizontal mounting was the thought that this could be a more efficient arrangement if more light is emitted from the sides of the spiral than from the top. On the other hand, in the horizontal design, only one side actually faces the contact frame. So perhaps the answer comes back to maximizing the UV reflectivity of the inside of the box in order to take advantage of as much as possible of the emitted light. That's why I'm planning to line the inside of the box with sheet metal (actually, aluminum flashing).