Concerning the Haste vs. Mastery discussion, you really should state clearly which variable you are trying to "optimise", because the issue you're discussing is not well-defined otherwise !
Average DPS is usually the default measure, and the one used by Simcraft (assuming accurate modelling, which is a different issue here). At constant average DPS, a haste heavy build will have lower DPS standard deviation than a mastery heavy one. This is because even though RPPM remains highly random at high levels of haste, getting more haste does indeed increase the expected number of procs and thus reduce the standard deviation of your DPS.
Now let's look at alternative measures and their effects on your haste/mastery preference :
- Median DPS (you get that DPS or better 50% of the time) : hard to tell, probably close to zero effect.
- 10% percentile DPS (you get that DPS or better 90% of the time) : you're being risk-averse (you want to maximise your DPS in a "worst-case" scenario), this shifts the balance in favor of haste.
- 90% percentile DPS (you get that DPS or better 10% of the time) : you're being risk-seeking (you want to maximise your DPS in a "best-case" scenario), this shifts the balance in favor of mastery (in defavor of haste, in fact).
Now this gets to more of a personal opinion on the matter :
- If you are in a progression context, there is nearly no situation where you want to be risk-seeking. And to be completely fair, your raid as a whole might be risk-averse, but your contribution to the global standard deviation is so small (in 25H at least) that you probably can be risk-neutral in most cases (i.e. listen to Simcraft).
- If you are in a farming context and want to maximize your chance at the best possible WoL rank, you should definitely be risk-seeking, and pray you get lucky ;-)
Important note : the difference in DPS standard deviation for both haste/heavy mastery builds is not insignificant imo, but keep in mind fight specific conditions favoring one or the other stat will flat out make them irrelevant. For example, even if you want to play risk-averse on a fight where mechanics heavily favor mastery over haste (say Durumu H, Destruction), you should definitely stack mastery in spite of everything I just mentioned.