Not much to add to the above really..
I would change your meta gem to Burning Shadowspirit Diamond
which will boost your crit damage
And only use the following gems:
Brilliant Inferno Ruby
Reckless Ember Topaz
(Use Brilliant Inferno Ruby as the main gem of choice, generally only follow the socket bonus if the bonus is 20+ intellect or around 30+ in another beneficial stat)
Agree with comments on glyphs, glyph of imp is probably the best glyph, as when you are running out of fire your imp will still be pewpewin'.
Hit is best stat after intellect so aim to be hit capped (I'm not atm but will be on Sunday when i get my new DM:V trinket !!
Other advice would be always try to use Shadowflame on cooldown if your near the boss, and if your running around due to fight mechanics and cant use normal rotation always try to Conflag, corruption, felfire, lifetape or even deathcoil while you run. Oh and use of Bane of Havoc on multiboss fights is a must, 15% more damage!
For more detailed info I would definitely recommend the post below on MMO Champ: