on stems and bars: most top-end Weight weenie stems are alluminium - getting them down to hundred grams. Carbon weighs about the same in the high end, but costs 3 times as much. Off course there are companies like ax lightness, but I'm talking stuff that's at least common market and is expected to be usable on a regular ridden bike.
Regarding frame responsiveness in relation to material: denser materials "ring" nicer - acoustics and ride feelings are correlated in a funny way. Hence bells are made of brass, not plastic and steel frames have that nice "zing" to them (unless it becomes overdampened by build/wall thickness). Alloy isn't as nice to the touch - and I love to ride alloy because it has an extra, communicative quality and a sense of lightness and purpose which I like. It has nothing to do with stiffness by the way. My ALAN is very whippy, whilst my Principia (which is somewhat of a crit bike) is hugely stiff.