It is a complicated discussion, and even more difficult to model "time off target" than time on target. What I've taken from the EJ threads is that it depends on which fights you've got on farm vs which fights you're currently smashing your face against, and what your role in each of them is. If you're sticking on Ji'Kun the entire fight, then I guess haste would probably trump mastery, but if you're on the egg team, the probably mastery is better. If your raid is still fighting Horridon, then haste isn't going to be as useful as mastery. Pretty sure Shadowcraft is really only capable of modeling what you do if you get to stick to a target.
Seconded. I'm 536 ilvl and I still go full mastery. The difference between full haste and full mastery is only ~1% on a patchwerk fight. Part of haste's value is the increase in RPPM mechanics. If you get bad luck, mastery is better. If you get good luck, mastery is better. Haste just makes your luck better over a very long time but you can still get screwed. It's only buying wow gold averaged over many, many fights that haste's effect on RPPM totals up to a DPS increase.
Additionally, if you're target swapping at all or losing uptime for any reason, mastery becomes better (poisons keep ticking harder with mastery and haste doesn't affect DP ticks while you lose more autoattack swings as haste). Almost every fight in ToT involves target swaps.
It's a very small gain to go haste in an optimal situation averaged over a long time. Proc RNG over smaller periods of time (like 4m-10m; a real individual boss fight) and real fight conditions (target swapping, etc.) however favor mastery. It's just not worth it at this point even in cheap wow gold full BiS gear. Maybe next tier the difference will be large enough to warrant going full haste even with real fight conditions if they continue to inflate gear at this rate.
I'm talking a difference of a few percentage points here too, so in the end, you can gear whatever feels best and still be perfectly viable. I prefer mastery for more consistent and RNG-resilient damage while sell world of warcraft accounts others may prefer haste for a slightly faster rotation.