The fact you compare Shadow Priest to Fire mages in the first place shows you dont know what youre talking about, the two dont compare.
Shadow Priests the most ramp defined spec in the entire game right now. Remember in the first 3 weeks of the xpac, you instant declined shadow priests? Hm, wonder why that is? Oh right, because theres ALOT of content where shit dies so fast they cant ramp up to their potential. Their aoe is incredibly mediocre, and only becomes acceptable in raids or high m+ where the mobs have such ludicrously high hp you get a full round of dots to tick after multidotting.
Youre compared a sustained dps ramp caster compared to a burst cooldown reliant caster, of course the numbers are gonna be different. You have above average single target, damn good cleave, and some outstanding burst aoe. You also provide a lust, a utility so valuable groups will literally decline everything else in order to get one. Your interupt is also 24 seconds compared to 45.
The problem with shadow priests is their scaling. I believe my shadow priests current stat weights are 1.0 int, .98 vers, like 1.23 mastery, and 1.47 Haste, 1.47 Crit. Obviously the value of those stats combined is way too high in comparison to some classes i.e. Destro locks who only have two stats at 1 while everything else is below that.
The reason for the "Reroll Fire Mage" meme is because you have no weaknesses. A common cry i hear is "But but shadow priests outdps us single target!" Sure, but you dont need to ramp as a fire mage. You also dont die if you lag for a second during combustion. You have a cheat death, an immunity, a blink/dash, and one of if not the highest burst potentials on aoe in the entire game with a simple rotation.
Please, enough of this "Fire is so bad we really dont need nerfs" crap.