I thought about the subject again and found some new questions I can't 
answer myself :)

The basic argument in the thread ABX provided a link to is, that CSG 
objects are optimized for the used transformations, so that e.g.

sphere {
	0, 1
	scale 20
	translate <-1,2,3>
}

is "translated" into

sphere { <-1,2,3>, 20 }

This is done to avoid the application of the transformation to the 
incoming ray, right? This is not applicable on referenced objects, as 
those would store their transformations separately.

Now I do see that this kind of optimization will work for simple objects 
like a sphere, but does it still hold true, say, for a rotated box? A 
sheared box? I don't know how the intersection tests are performed 
internally, but I could imagine that there are (a lot?) of combinations 
of objects and transforms for which it is not possible to perform 
optimizations as above. So in that cases, the transformations would have 
to be applied to the ray, wouldn't it? And applying the transformation 
to the ray is a matrix*vector operation, right? So once you mixed one 
transformation with an object which can't be optimized for that special 
transformation, you will have to do the matrix*vector stuff, no matter 
what any other transformations are applied to the object. In those 
cases, the argument against referencing does not work.
Can anyone tell me how much of the usual intersection tests benefit from 
this kind of optimization (I know that this is scene dependant, but just 
to have an idea)?

Thanks for any pointers and sorry if I'm talking BS, just tell me :)
Florian
-- 
camera{look_at-y*10location<8,-3,-8>*10}#local a=0;#while(a<999)sphere{
#local _=.01*a-4.99;#local p=a*.01-5;#local c=.01*a-4.995;<sin(p*pi)*5p
*10pow(p,5)*.01>sin(c*c*c*.1)+1pigment{rgb 3}}#local a=a+1;#end
/******** http://www.torfbold.com ******** http://www.imp.org ********/

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