I have included Sunfire in my siren book a few weeks ago, but I could not try it yet. Today will be my first try. I will tell.
What I like from this is: being the siren a fair flexible mage, this move does not define your strategy too much, and you can act from there according to your best interest against this or that move from your oponent.
I mean, if you see a jokthari, and you begin with sunfire+crystal, you gain time to see if he plays lair or not, so decide if you play turn 2 echo or turn 3 elemental...
As pointed above by arkdeniz (but for his necro), another option for siren could be Sunfire+Idol+shallow sea... that will force anyone but a necro to attack your siren. But...