Something I realised on closer reading of the Optimization article is that there's actually a very good way of debugging the number of lights you have:
The number of lights in a space affects performance as well. Try not let any object be lit my more than two lights. You can get a feel for how many references a light affects by toggling the "L" hotkey. You can also look for lighting issues in a space by right clicking in the render window and selecting "Render Window Properties". Go to the "Shaders" tab and check the box that says "# of lights". Fig 7.3 shows an example space that has too many lights. The colors represent the number of lights hitting a each reference. Green represents an unlit piece and red means there are too many lights on a piece and performance issues are likely. It is good practice to make sure nothing shows up red when lighting your dungeon.
The thing is that the problem isn't just the number of lights in a room, but it's also compounded by large objects - in the case of a large cave, or a large castle etc. you run the trouble of having
one object receiving too many lights, which is the main part of the problem. The game doesn't actually care how many lights there are in a given room - merely how many lights each object is lit by (which are added up by the shader during the draw call). That would normally be a pain in the neck, but the debug shader actually works really well

Of course, don't forget that shadowed lights are a special case (I'm guessing there's only a fixed amount of buffers for drawing shadows, which get distributed amidst the visible shadowed omnis. Those you do actually need to hide, so you can't see more than four at a time.
Hope that helps!