according to the simulationcraft (which im in the process of figuring out so i can run it on mine, and all of yours' gear)
puts 20/51/0 as the top spec about 100 dps higher than 0/53/18
now you might all say.. zomg gnomer your post about 0/53/18 being the awesome sauce. well, i'll explain
first of all, 20/51 requires slow to be cast on the boss, which means you need an arcane mage to cast slow. this has to do with torment of the weak, causing 12% more dmg with fireball. thats great, but arcane dps is well... not so great. it would require 10 mages in the raid, to make all the mages spec 20/51 and one arcane mage (unless there is annother snare that can effect raid bosses, and as of now i don't know of any) to make the raid dps higher from the mages.
20/51/0- 5283 dps
0/53/18 -5177 dps
20/51/ without torment - 4902 (this is without having an arcane mage in the raid)
18/0/53- 4775 (just to throw the frost numbers out there)
52/19/0 (casting slow) - 4262 dps
so, at most, we will have 4 mages in the raid (its possible, but not probable) so, the highest mage output for 4 mages in the raid would having everyone spec ffb. the having one mage cast slow kills their dps (by 900 in comparison to ffb).
this is theoretical, and assumes that there is no imp scorch being thrown up (in both specs). even while throwing scorches, (i would say is about a 200dps loss) ffb is still at around 5k dps (in reality it will be closer to 4500, however could spike much higher depending on how often you crit (and lower too)).
so, there is some analysis on the numbers. just to throw this out there, deep fire without totw is still a viable spec, and if you don't have a lot of crit rating will be just slightly behind ffb. Also, you need 3% more hit (13% in raid) with the deep fire spec, because you don't have any talents buffing your hit, so thats an additional 76 hit rating to aquire, that could otherwise go into sp or crit.
and annother note, arcane blast is not part of the arcane rotation, which i find interesting. its slow, fireball, ab, fireball, ab.. etc, with am when it procs for half time am. but yeah.. arcane is broken right now. i hope it gets fixed in 3.1, if it does, and totw works nicely (its kinda bugged now) deep fire may actually surpass ffb.
highest raid output of your mages core= every mage speccing 0/53/18 (unless you have 10 mages, but well... honestly? i would bitch slap the raid leader who let that happen)