Everyone's a Crit-ic

Apolgies for the delay between my posts again, but I'm sure the world isn't going to come to a standstill just because I stop babbling on.
Things at work are going to get a little more back to normal from this week, so my frequency of posts should go back up too.

Anyway.... Crit vs AP... Discuss.

My DPS is usually at the top of the meters for an overall run, but that doesn't necesarily mean that I do the most damage on bosses. Particularly on ones like Heigan, where I can't even send in my pet.
But generally speaking I'm happy with the amount of damage that I'm doing.
My question is, how do I do more?

At first I was stacking as much AP as possible, and it was working, I saw a steady increase in my DPS. Until I reached 4k AP unbuffed. At that point, my DPS seemed to level out a bit, and even when buffed up to 6k+, I didn't necessarily notice the major improvements that I would have expected.

So I switched out as many gems as I could for +Crit instead of +AP. Which now gives me just under 4k AP (Unbuffed), and about 28% Crit (Unbuffed).
Lookign at my last ReCount meter from Naxx, it shows I averaged 36% overall Crit, and yes I was 1 Million dmg above 2nd place (overall), but my DPS hasn't really moved at all.

I've also picked up t7 Gloves and Chest, so my pet is now 5% dmg higher (which should equate to about 50DPS)
Although I'm still stuck with my Blue helm and shoulders, and both rings, and 1 blue trinket. Unfortunately, the only place I can pick up decent upgrades for all of them is Raids, and my luck on drops hasn't been too good.

So, any ideas? Should I be stacking Crit, or should I go back to raw Attack Power?