A method that might improve perfromance

I know from my recent year of beta'ing this game that their was one thing that bothered me a lot. This is a question more for the programmers of Linden Labs. Lets take this for example:

I make a sphere, and a box. I put the sphere inside the box. Now I can't see it. Simple so far. But when I view it with my avantar, and the information of thoes shapes is proceesed in my GFX card/CPU, what is exactly processed? The polygons of the box, or the polygons of the box, and the sphere? If it is both, then this would be a great oppurtity to implement a Subtract and Union feature into SecondLife. I know this was posted a couple of times about enhancing the building features. But I am talking about it more for improving the performace of the game locally on our computers. If it is not and only the box is processed (or what can be viewed by the avantar), then that is great, and this post is usless.

My experience with graphics cards says that, most likely, both the sphere's triangles and the box's triangles get sent to the graphics card, but the triangles making up the sphere get discarded very early in the rendering process, for a minimal slowdown.