I myself have followed the theorycrafting and discussion on the upcoming changes to the arcane tree with great interest. But there was one thing that always struck me as problematic: everyone seems to make up some calculations on his own, or using some rawr simulation while not actually providing any information on how they designed the encounter.
To clarify this for myself and for others, who might not have considered the whole picture, while undergoing their calculations I decided to summ up a bit of facts and information as well as my interpretations on mechanics.
I noticed, that a lot of people have statements like "12% more damage" or "the critrating I lose while trying to reach the hitcap makes up for the damage I might possibly gain". This shows in my opinion, that there is indeed need for a summary, because missinformation like that is resulting from someone only looking at the picture from one angle.
Let me start with summarizing the damage-enhancing changes to the fireball-spell in comparison to the Frostfirebolt.
1. Glyphs:
1.1 What many people seem to ignore is the fact, that the only thing changing with the glyphs we will use is at the same time the thing that makes up for the lost hitrating due to the lack of Precision in the 18/53/0 specc : We will use the Fireball Glyph, which will increase the critchance of the Fireball spell by 5%, opposing the 2% and 2% more base-damage to the FFB-spell of the Frostfirebolt-glyph.
Note: Fireball will have a 3% higher critchance than FFB due to the different Glyph.
2. Talents:
2.1 The FB and FFB share the same firetalents when it comes to DPS, although it should be noted, that the 18/53/0 build compromises the effective range of our firespells and one other talentpoint (where that would be is up to personal preference) in favor of 5/5 improved FB.
2.2 The really interesting differences start at
"Spell impact" vs.
"Piercing Ice". If I am not mistaken about the mechanics of these talents, they will actually increase the damage of FB and FFB by 6%. So whatever specc you should decide to use, your main-nuke will actually have its damage increased by 6%. Seeing how fireball does more base-damage,as well, as the better scaling with SP, while still having the same casttime, "Spell Impact" will give more DPS.
2.3 The next important talent that is going to be different is
"Precision". We will have to make up for the 3% hitchance that we lose by speccing for 18/53/0 this is where people say "See? here it boils down to the same anyways", but as we will see, this is not true at all. If we assume, that we have to replace 3% hitchance, we will have to get about 79 hitrating more than before, because 1% hitchance is 26,23 hitrating.
If we now assume that we have to replace 79 critrating with 79 hitrating, that would only lower our critchance by 1.71%, due to the fact that 1% critchance equals a critical strike rating of 45,91.
Note: Fireball does by now still have a 1.29% higher critchance (3%-1.71%) than FFB, while having the same hitchance. This doesn't even consider the fact that you might have been slightly overcapped before anyways. As well as it has a higher Base-damage.
2.4 Eventhough
"Icy veins" is a 20% damage boost on the average bossfight for about 40 seconds, it would indeed require a simulation, to define its actual boost on a certain fight, so I will leave it aside for now
2.5 The lack of
"Frost channeling" hurts the mana-effectivity a lot, considering the higher base-manacost of fireball.
"Arcane Concentration" on the other hand will make every 10th fireball free, so we have a little bit of mitigation there, as well as the fact, that no other spell in the FFB rotation actually benefits from "Frost Channeling", while every spell in the FB rotation does from "Arcane concentration". Bottom line is, that the mana-consumption of the FB specc won't be as inferior as predicted at all.
Note: The mana-cost of FFB is only 72% of the FB spell. 80% when considering clearcasts. Its not that bad after all though
2.6
"Focus Magic" Due to the huge benefit firemages get from crits it would be quite usual if there were 2 mages in a raid, that they'd buff each other with "Focus Magic". Considering the huge raid-buffed critchance of any mage, this would result in a nearly permanent 6% increased critchance.
Note: The FB-mage has by now 7.29%, or slightly higher, critchance more than a FFB mage to crit with his main nuke. This has direct effect on the DPS, along with indirectly by triggering
"Hot Streak" more often than before.
2.7 The crit multipliers of FFB and FB are different (3,346 and 2.583 after the t7 setbonus and chaotic Meta) so in order for them to do the same crit damage FB needs to have round about 1.295 times the damage before the crit than FFB does. It does, due to
"Torment the Weak" and the inferior SP-scaling of FFB up to a SP of 2056
f1(x)=(780+x*1.0071)*(1.1+0.02)*1.06*1.03= 953.8+1.23*x
780 is the base-damage of FFB and 1.06 along with 1.1 resemble
"firepower" and
"Piercing Ice" x is your SP and 1.0071 is the SP-coefficient for FFB the 1.03
"Playing with Fire" 0.02 is the FFB Glyph This gives a simplified calculation on how much damage FFB does at a certain amount of SP.
f2(x)=(1010+x*1.15)*(1.1+.06)*1.12*1.03= 1351.56+1.54*x
Same here, though 1.15 is the FB SP-coefficient and 1.12 is Ttw. This Is the damage for fireball.
You will notice, that up to a given amount of 2056 of SP f2(x) divided by f1(x) will never be as low as 1.295. So if I calculated the damage according to the actual game-mechanics, fireball with the enhancement of Ttw will always hit and as long as you have less than 2056 SP also crit higher than FFB. If you manage to go over 2056 SP your FFB will crit for more damage than the FB.
Summary:
-FB-specc's main nuke has a slightly higher critchance increase over FFB than 7.29%
-FB-specc's main nuke has a higher hitdamage, and competitive critdamage
-FB specc will be more mana consuming than FFB, but in a way, that shouldn't concern any high-end mage
-We have to keep in mind that we cannot only look at the main nuke to determine the total DPS-increase of the firespecc, but we can very well say, that the main nuke does far more DPS
-FB-specc's DPS-curve won't be as spiky as FFB-specc's, because of the lower dicrepancy between base-damage and crit-damage
-We lose Icy veins
*edit* horrible typos (I'm just not english

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*edit* Thanks to Manly for pointing out my flaweed calculation on The FFB Glyph and The main nukes, it should be ok now, and the basic statement hasn't changed.