Sunaseni has it pretty much there.
What I did, as soon servers came back online, was to take my mage to the reforger and make Spirit on my gear into Mastery (I highly recommend this- it isn't on topic but I don't care).
What I found in random dungeons was that in the now-rare longer fights (things like the Gauntlet in HoR or stupid terrible Tribunal in HoS), arcane DPS starts at a very high very silly number then goes to a place a little above where it was pre-patch. In the fire spec, damage was consistently lol-high.
The problem with both specs were the super-duper high numbers; tanks couldn't keep up with keeping aggro and healers weren't able to heal us ranged casters after we took pretty much all aggro with just a few cycles of the normal arcane rotation or watching DoTs and RNG and then King Dred eats you or something. With lesser geared/quality tanks and healers, we (this was in a few different runs and including a filthy aff lock, boomkin, another mage, and a couple of elemental shaman) were pulling aggro on trash mobs, with just doing our regular things.

Also take in mind, that this nerf is more of a normalization. This is also before the Uh-Oh with our glyphs has been fixed (a whole bunch of prime glyphs got initialized as major ones; they are Fbolt, Fball, FFB, AM, ABarr, and Molten Armor- so the ones for three of our four main nukes, two arcane side-arms, and the armor that makes us have moar deeps are not working as intending and we're still bringing in silly numbers).