Horses are worth 2 offense, not 1. Stabling your orc elites should add slightly over 20%.
Your rarely 100% b/e going into war, and there is decreased building efficiency for most buildings the more you make of them. You probably won't get 17% off those training grounds.
With elite values as they are now, I think you always mount your army before training grounds. There is the fact horses add NW, TG's do not, but pretty sure you still stable even orcs.