[Solone]

Through the fog of my Sunday head, I'll try to answer a few points as best I can on this crappy handheld device thing... Regarding rendering on the GPU, I actually think the optimal solution is letting the engine use both CPU and GPU in tandem. At least that's what I recall reading in some tech forum.
Rendering just the character will definitely speed time up, and can often make it much much easier to have control over the lighting. However, you are then missing out on all the reflected incidental light that plays around a scene and brings out all the interactions between form and shadow.

Defeating graininess is really just a case of letting the engine run longer and or to a higher completion ratio and render time. Iray will stop when it hits whichever cap comes first, so you can then just up the max time/samples and click resume.
Under optimisation, there are two extra settings to take note of.
Caustic sampling and architectural sampling. They will eat up your render time and aren't always effective but to give a brief idea;
Architectural is great for indoor scenes, light interacts with surface on slightly more complex level creating better definition in shading on the figure/scene.
Caustic, I'm still figuring out, but it seems to render specular highlights a little clearer and more accurately. It also tends to help with scenes involving clear surfaces such as glass or water.
Anahki, to answer your question, I dunno! There is a "render priority" tab on objects or surfaces (both? I can't recall) but I don't ever use them. My slightly obsessive nature means I want everything at full quality at the expense of time. Probably not an ideal trait for someone who deals in sequences but, eh, it's just how it is. I wouldn't worry about these settings for every pic, buts it's always worth checking to see how much difference it'll make. Sometimes it's really not worth doubling your render time for that single highlight on that out of focus glass bottle at the back of the scene.
If people are interested, I could post some samples of renders with and without these settings.
That was tooany words for a Sunday. I'm off to eat an animal then lie down. Happy Sunday freaks x