I have only played one game with that item and it was in a custom i left however It appears that the movement speed buff will only work if you smite before killing the small creeps which seems pretty pointless when you have to sit for 2 more seconds to kill them.
I've also seen some pretty variable results from smite killing large monsters. Sometime you get the speed buff, sometimes not. In my experience, killing the other mobs is irrelevant.
Take wolves camp for example.
Focus down the the big one and before smiting it, get a baby wolf down to one AA until it dies (if you want to clear them all, get both babies down to 1 AA). Then kill the big one with smite, aa the baby (or 2) and gtfo.
Should leave one minion for counter jungling to set the enemy farther behind. That's my personal preference anyway.
