I don't think Fast/Haste stacks. I didn't know Invis stacks, but Temper and Saber certainly do stack. If you keep casting Temper and Saber on a Knight with some strong weapon, he can eventually cause thousands of HP of damage. I was fighting imps/goblins, and the Knight did 3765 HP damage against an enemy, and after casting Temper and Saber for a few more rounds, he did 5368 HP damage against another enemy. This was in the Origins version.