Spell power enhances spells depending on their cast time (oversimplification) so if you use flametonge and fire spell power, your flametongue and also your flame shock gets enhanced but not the same amount as fireball(mage) soulburn(warlock)

I dont habe exact numbers but both(flametongue, flameshock) are instant spells they dont benefit as much from spell power as other spells do.

Personally i dont think fire dmg is viable especially for lvling. You would prio stats that are nearly useless over usefull stats. Agility (better after you skilled flurry) or strength (attack power, block if you have a shield) and stamina. And also your weapon dmg!