This is the main reason why I personally believe FFB will still be as viable as 18/53/0 in the long run. EJ has calculated the new fireball spec as doing %1 more total damage, that's not worth having to regem and/or put on more hit gear, imo. The other factor, of course, is mana efficiency.
"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."
With the influx of WWS reports as our data, it looks like arcane and arcane/fire are putting out almost the same dps. Although arcane seems to have a slight advantage in 25 man raid situations, owing to the higher amount of mana regen.