I love my merc heals in pvp. It's a really a fun class to play and takes alot to master. Not everyone can survive initial burst. That being said, I put out way more heals per second (probably double) on a Sorc. I stopped worrying about what's best and stuck to play style I liked. We could use a buff but Merc heals can hold their own in right hands.
Small buff suggestion - Add HoT on Kolto shot that stacks or kolto shot increases effectiveness of kolto shell by some percentage every stack.
-Lotto