Before Whaat provides you with a detailed description I can shed some light, what else.
1. Ultimately, emission scale refers to the rate at wich an emissive material will shoot rays.
2. This numeric value is variable depending on the time of day and is displayed in the render log before the render starts. I tend to use a value of 8,000 lumens for outdoor emitters, it's slightly more than the sky can provide I think.
4. There's something special about the unit one would choose in order to scale emission: some units are dependent on the emitting square surface, some set a constant value for the luminaire independently from it's dimensions. I am not sure but I believe that only lumens is absolute, ie scale-independent.
So yes, that's how you set two emitters to emit at the same rate. However concerning the CPU/GPU usage I assume, it also depends on the exposure of each emitter to the camera. Consider one simple scene with two emitters at the same "lumen rate"; the one the less obstructed is by essence the most efficient.
That's where the artist's eye plays a role also, in estimating visually the efficiency of the contribution of each light to the scene. You may indeed want to tune that most efficient light down (and eventually put it on its own layer - that's not necessary though) so it doesn't over run the evaluation of the other light emitter, wich was primarily as powerfull but more obstructed to the camera (less efficient).
Hope it helps
PS: you can refer to the Indigo Technical Reference, page 97 for more informations, and to this link:
http://en.wikipedia.org/wiki/Illuminance