I'm using a very simple macro for Soulburn + Soul Fire. It's:
/cast soulburn
/cast soul fire
Like any decent caster, I do anticipate cast intervals - especially handy when spamming SBolt. I've noticed that when I hit this macro at the same cadence as I would a normal cast time spell, I'll end up casting Soul Fire without the Soulburn effect. Anyone else see the same effect, or have thoughts on how to fix this?