Drill master does increase speed of whole army, I think, you just have to have drill master equipped on your general at the end of the turn. Then at the beginning of the next turn, everyone in his stack gets movement bonus. I don't know much about black stallions, but according to this page http://noctalis.com/dis/totalwar/m2tw_govretinue.shtml they should be transferrable with similar effect as the drill master. Having these two retinues along with the general traits that allow movement bonus, and you literally have an overnight delivery guy.