i remember that someone from the list wrote that using [expr] or [expr~] consumes more cpu than doing the equivalent with 'conventional' objs. but i can't remember the reason and how much the difference is. maybe you'll find it by searching the archive.
i once did a small benchmark ... seemed that expr and the pd objects are about the same ... not sure about expr~, though ..
still, if you are about tweaking your system to hell, the best solution is k_cext, since it does your task in native machine code ...
cheers ... tim