Over a long fight, the RNG evens out usually.

I looked at the #1 Fire Mage dps for 25H Ultraxion, the #100 fire mage, and the #200. Pyroblast! (with hot streak) accounted for 12.4%, 12.9%, and 12.3% of the damage respectively.

A bigger deal with crit might just be the actual crit hits. The absolute top dps'ers are getting just about 50% crit on their fireball, which is the top dps spell. The others I mentioned above were around 41-42%.

Fire tends to scale much better at the end of an xpac because of how well crit scales on their spells as well as the hot streak mechanic.

Also, the absolute top dps'ers are getting maximum DPS out of combustion, compare 2.3M combustion for the #1 dps and 930k combustion for the #200 parse. Conclusion: Maximum dps on a Fire mage is both skill and gear dependent.

I play a fire mage for an alt and the randomness can be quite annoying. One fight you are destroying the meters and the next 3 you look like a fresh 85. I have done entire boss fights in heroic 5-mans without a single hot streak proc as well as a few raid bosses.

Jeremie, that definitely happens at lower levels of crit, like the 15-25% range. Once you start getting above 40%, it tends to be much more consistent in terms of the feel of it. (that's 40% raid buffed, not just your own unbuffed crit %).