Kill Command does NOT activate the GCD. However, it cannot go while casting steady shot, and if it goes while casting auto-shot (the last 0.5 seconds of the auto shot) it will delay the AUTO SHOT by 0.5 seconds after the KC goes off.
Steady shot is enhanced by rapid fire and other speed increasing effects, however you have to consider the last 0.5 seconds of your attack speed will still be auto-shot. So for the most part, unless you have a huge amount of increased attack speed (or a REALLY fast bow), your steady shot wont delay your auto shot.
As to the question by herbnosis, I think the problem is you are pushing the button to start steady and pushing the button when KC is up (I'm not there, but I'm guessing thats what you do). The beauty of this macro is you can spam it faster than an MP-5 can fire, and it will still keep a steady-auto rotation, weaving in KC if it's up. KC will only fire right after steady or right after auto-shot, and if you're mashing the key fast enough it should not delay either (except for maybe the time it takes to depress the button, which shouldn't be long if you're mashing the key as if it's a potato that you're going to put gravy on). So throw your key a beating, as if it were a disobedient kid in 1930, and use the macro to it's fullest.