Regarding my comment about devaluing mastery, it's kind-of complicated to explain & this is based off napkin maths so bear with me here & I'll try to explain.
Imagine for a moment that MB had no cooldown and was infinitely spammable. There are still only a finite number of MBs you can get into the 14s window. This is essentially what Mastery is giving you at a high enough %age. Even with Mastery as low as it is on my gear I'm occasionally seeing this happen during AotE usage, this will be the case more often as haste increases pet attack speed and thus MB proc chances. Thus more Haste = less need for Mastery.
If MB was our only ability then increasing the frequency of what I'd call MB saturation as described above would be the gearing goal but we have other abilities, mostly DoTs, that are a net increase to our overall DPS. To maximise our DPS, we need to be doing something other than endlessly spamming MB*. These abilities take GCDs to cast so again, less need for our mastery to be providing enough MBs to fill every GCD.
*This is where the napkin maths come into play. I'm not 100% certain of this statement, it may not be true for pure ST dps.
Hope that made some sense!