For weapon and defense skills, which just give a flat bonus (like 10% damage per level), you can stack them as high as you want and still get the benefit of it. Many enemies have something ridiculous like 9 or 10 willpower and bodybuilding as early as the Cyseal area. In my last game, one of my characters has 9 Tenebrium (I had 2 pieces of gear with +tenebrium)
