So I just respec to MM And it's really good so far.
My question is , Should I cast Aimed shot with Steady focus which is 1.82 , Or just instant cast / RP
What will be the best?
It really depends on your gear. With a 1.82 cast time, it may not be worth it to hardcast Aimed Shot until you gear up a bit and acquire more haste - you would probably lose a bit of DPS actually. However, after this next Tuesday Aimed is getting a substantial buff, making it even with or better than Arcane Shot at most gear levels witht he delta only growing wider the more haste you get.
Also, bear in mind that the differences between Aimed and Arcane MM specs on live is very, very small. In 5.4 it will be a bit wider with Aimed taking the lead, but it's still a relatively small gap of around 3% if I recall.
You only want to hard cast Aimed Shot if your target is over 80% health or if your Aimed Shot is below a certain cast speed. I've been having success with 1.2 seconds but others have said they hard cast at 1.4 seconds.
Regardless you should always make sure Steady Focus is up.
And still others say 1.6 seconds. It depends a lot on your gear. With 5.4 in 4 days, the best I can do is recommend self testing to see what works best on an individual basis. I know for myself, dropping Arcane altogether and hardcasting all the time, should be a noticeable increase next week.