Only recently been toying with Demo for Heroic Maloriak and ran into the same visibility issues. I ended up trying what has been suggested above: turning off my own spell effect emphasis and projected textures. These help but it really comes down to getting comfortable with moving with the add tank. Have him marked, watch your debuffs like a hawk, and try to plan out safe spots for you to move to next. Healers should also realize that you will probably have the hardest time avoiding some ticks of the DoT as well as the small amount of damage that hellfire does when channeling.