rayman.. as far as efficiency goes.. when i'm doing a scene-in-a-cube.. i'll sometimes get as much as 4500% efficiency.. and when it's a just a scene out in the open.. i'll get 400 or 500%.  Efficiency is a measurement of how easy it is for lux to find your lights.  It's a measure of how many light samples on average each sample has.  I've found there are a lot of different details on how to get your efficiency up there.

1.  If it's an interior scene with exterior lighting(i.e. inside a house w/your lights outside the house) use portals.

2.  If it's a simple scene like just model/clothes/hair/backdrop.. then put it in a cube and make sure that no portion of your light is outside the cube.

3.  If there's a lot of glass in your scene and you don't need the refractive effect of the glass; turn it to architectural.

I've attached a screenie of a simple model/clothes/hair/backdrop in a cube render that I did as a burn-in render for my new computer.  Note the efficiency.  Almost 3000%  That efficiency combined with decent s/s rate added up to a c/s rate of over 2 million c/s.  That equals a render clearing up in a couple hundred passes instead of a couple thousand.

Without adding the gpu into the equation; my effiency is usually half what a plain cpu-only render is.  Still respectable though.