From me experienced
reforge mastery to haste = dps increase vice versa reforge haste to mastery = dps lost
reforge critical to haste > mastery
I have tried to maximize mastery at 51% it suck ! never try full mastery ! just go as much as haste u can.
Somethin' tells me, that the person above completely ignored the fact, that OP has T13 4p. Might even go this far to assume, that his "experience" comes back from the times when Lava Burst reprocs didn't crit.
Normally with T13 you'd only slightly prioritize Haste, but generally go with the stats "as long as it's not Crit, it's good". Now I as far know, on Molten the Mastery procs don't behave as separate spells, they don't proc Rolling Thunder for example. How much that affects the value of Mastery, though, it's really difficult to tell.
And truth remains, that Mastery is king for AoE situations.