Shapes and sway for detail props is already available in the sdk source code, and it's an extremely simple task to enable them, apparently. It would be nice to be able to use this functionality to make more interesting and better looking detail props for materials. Info can be found here: https://developer.valvesoftware.com/wiki/Detail_prop#Shapes It'd be cool if we could do a few more advanced things in the future with detail props too, such as erasing ones underneath buildings and turrets.
Warning: Shapes place more load on the GPU, while sway places more load on the CPU. Use both as sparingly as possible. (The Dear Esther team have developed GPU-accelerated sway which is faster and more realistic, but their code is unreleased.) Hmm..
"Excessive use" of detail sprites/props boils down to two things: the density and the fade distance. Assuming the mapper understands a few basic things I'm sure shapes and sway will not have any noticeable performance effect. I believe you can disable them for dx<9 cards, too.