optimizing speed of SMIL

If I want to shrink a rectangle, ellipse or image toward its own center, I
can either use four <animate> tags to adjust its x (cx), y (cy), width (rx)
and height (ry), or I can use two animateTransforms (with additive="sum" )
to scale and translate.
 
Does anyone have a sense of which would be more computationally efficient
given current browser implementations?
 
Cheers
David