My by-the-book answer is keep stacking it. My experimental answer is there are fights where it MAY make sense to start stacking Mastery even at the expense of Haste. Fights with multiple targets, significant add phases, and extended Eclipse can all start to weight Mastery higher than Haste past that break point.
My current reforges are weighing Mastery higher than Haste after the 1929 break point but, again, I'm experimenting. Also, if/when you start pushing Sinestra, this experimentation can become even more extreme, pushing Mastery priority even further up and Haste further down to even the 1423 break point (due to the 100% Haste buff on Sin during the only dps phase that really matters).
All of that said, I can't stress enough that this is experimental territory and that the good, safe bet is to continue stacking Haste until it comes out of your ears.