Hi, I have been playing MM the past two weeks and quite like the direction it is going. In fact, this is how I wish MM was played in Cata. I was never really a fan of hardcasting Aimed Shot but I went with it because it was best and it was very risk/reward which made it quite thrilling on fights where you could find the time to flatfoot to charge up that Aimed Shot. These days, it's recommended to hardcast Aimed Shot during haste effects like Bloodlust and Rapid Fire but I feel this isn't necessarily true anymore. In fact, the only time you'll want to hardcast Aimed Shot is during Careful Aim. Using Arcane Shot as the focus dump of choice for 80-0% makes MM less clunky, more enjoyable (as enjoyable as spamming an instant shot can be) and easier to pick up since you won't need to keep all the conditions in your mind. You shouldn't have to ask yourself something like, "Is my Aimed Shot under 1.4 seconds? I should start using that instead of Arcane Shot then."
First, I'm going to go over the priority list and action list used in simcraft to determine the difference.
The most important thing in this is "#actions+=/aimed_shot,if=buff.rapid_fire.up|buff.bloodlust.react" which removes the action from the list and "actions+=/arcane_shot,if=focus>=60|(focus>=43&(cooldown.chimera_shot.remains>=action.steady_shot.cas t_time))&(!buff.rapid_fire.up&!buff.bloodlust.react)" by removing "&(!buff.rapid_fire.up&!buff.bloodlust.react)" to allow Arcane Shot to be cast while under the haste effects (Thanks Don Vit Corleone)The line removed ensures you only hardcast Aimed Shot during haste effects (Rapid Fire and Bloodlust) and you will cast Arcane Shot during haste effects.
The important part is this is actually a gain of ~50 dps.
Google Docs of data
Looking at Fervor first, you can see Aimed Shot during haste effects is 226203 dps and Arcane Shot during haste is 226248 dps, a gain of 45 dps.
Dire Beast is a similar story; Aimed Shot during haste is 228764 dps while Arcane Shot during haste is 228891 dps, a gain of 127 dps.
As we all know, haste effects increase the amount of Autoshots done but Aimed Shot prevents this benefit since it stops the Autoshot. With Arcane Shot dumping, you allow Autoshots to continue while having a 1 second GCD ability go off. A reason I've not been a fan of hardcasting Aimed Shot is because it basically reduces the effectiveness of your mastery by 50% since you lose the chance to proc Wild Quiver from Autoshots.
Looking at the charts on the Dire Beast sheet shows a gain of ~11 Autoshots, 15 Arcane Shots, 5 Steady Shots, and 5 Wild Quiver procs at the cost of ~8 Aimed Shots.
The entire premise of this is that it is a small change to make since there are so few moments where this even applies. For example, this doesn't matter for the start of a fight since you start in Careful Aim so you will do Aimed Shot regardless. A better example would be the start of Phase 2 on Heroic Lei Shen; my current timing allows me to pop Rapid Fire as soon as the boss is active but he's at 65%, meaning I will use Arcane Shot as my dump instead of Aimed Shot.
tl:dr: Only hardcast Aimed Shot when the target is above 80% health.