Why is my dps so low on heroic dummy?

I've recently ding'd 80 on my mage alt. I've read a guide provided by Ensidia  about how to play mage at level 80 ready for raids. I've got plenty of raid experience and characters so I don't think i'm missing some massively important in my rotation or spec. But when I test my dps on a heroic dummy I can barely scrape 2k without using cooldowns, which by my standards isnt accept even in naxx 10. I'd like to ask your advice to see if I'm doing something wrong or there is a problem with my spec etc. Here is a link to my armoury.

Before you mentioned it yes, I know i'm not hit capped and this could be the reason i'm only scraping 2k dps but when testing my dps on the boss dummy for a whole minute I only missed 2 fireballs which I dont think would have increased my dps to anywhere near what I want.

I also have a low crit with fire spells, I'm thinking this might be the cause since when I crit with pryo blast or a fireball my dps can shoot up to 2.5k.

Currently my rotation is Fireball spam until hot streak, use pryo and start again, all the while I keep living bomb and 5 stacks of scorch up 100% of the time. I've tired a rotation with fireblast included but that made my dps even worse dropping it to 1.7k.

testing  dps to a target dummy wont be accurate...not by a long shot.