This actually doesn’t matter that much. Stars is 2 RPPM (scaling with haste), which means the average proc interval is already 30s (before haste). RPPM’s formula for “time since last procable event” caps at 10s, but so long as you hit the target at least once every 10s, you’re not actually losing anything as far as Stars is concerned.
It’s really multi-target that harms Stars so much. Stars procs on a random target in range, and nearly the entirety of Stars’ damage comes from the buff from stacking it. A normal random-targeted damage proc, like Gushing Wound, would deal similar damage against 10 targets as it deals against 1, since the procs simply split up. But with Stars, you’re going to be doing only a fraction of the damage against 10 targets as against 1, because each target will take 10 times as many procs, on average, to cap out the stacks.
The reason Stars is so good for Havoc is not that they hit faster. RPPM is specifically designed to normalize proc rate across wide ranges of attack/cast speeds. A person chain-casting Scorch, a person chain-casting hardcast Pyro, a person Fire Blasting every 8 seconds, and a person dual-wield auto-attacking every 0.5s will all average the same procs per minute with RPPM.
The reason Stars is so amazeballs for Havoc is because of the synergy between RPPM effects, Furious Gaze, Demonic Appetite, and Eyes of Rage. The average procs per minute for an RPPM effect scales directly with haste (Stars is 2 RPPM, so if you have 50% haste, it’ll average to 2 * (1 + 0.5) = 3 procs per minute). Furious Gaze gives incredible amounts of haste after Eye Beam (~17% per trait at 480 itemlevel, so ~51% with 3), and Havoc also gets a bonus 25% (multiplicative) haste during the (Demonic-triggered) Meta following Eye Beam. Eyes of Rage reduces the cooldown on Eye Beam, allowing a higher uptime on both Furious Gaze and Meta’s haste bonus. Finally, Demonic Appetite provides souls to fuel Eyes of Rage, and is also RPPM and thus scales at the same incredible rate as Stars.
Net is that Havoc can maintain something like 65-75% uptime on Furious Gaze, and will often have something like 110-125% haste while both it and Meta are active, meaning that 2 RPPM becomes closer to 4.5. During Bloodlust, that gets closer to 6.
That means an HDH will cap on Stars stacks much earlier in the fight, and will then be procing a full 10-stack stars twice or more as often as other classes.
I suspect so, though I don’t have the effect to test it on my mage. I know it Expedient (12% haste) bonuses the haste from Brigand’s Blitz (also an Azerite trait) on my rogue. I don’t see any reason to suspect that Masterful wouldn’t bonus Blaster Master’s bonus in the same way.