Honestly anything below T6 doesn't mean much in this context, enemy HP is almost doubled each jump of torment. Of course you can get by with a lot less on T5 compared to T6, that is why so many other builds still work on T5 that are completely dead on T6. I assume my demon hunter can do T5 with only 3 sentries as well, but I pretty much always get 4 out on the average elite/champ pack on T6 (unless it's a typically low health mob). And on the flipside I can pretty much always get 5 out on an elite pack where the elite mob in a naturally bulky type (and it's guaranteed if it has shielding).
Of course stuff like CDR factors into this as well, my demon hunter has around 40% because the way my gloves rolled so I can pop out sentries at a pretty fast pace in the first place.
Also for what it's worth steady aim is not useful "all" the time. It's pretty much trash when you encounter mobs that try to bum rush you, so basically nearly all act 5 mobs and some other guys. It's easier to maintain with vault, but, most people on T6 don't run with vault since smoke screen is infinitely better for survivability. The only way you can run both vault and smoke screen is by giving up another DPS increasing ability.
I guess my question would be, why are you soloing T6 if it takes so long to clear that you are getting 5 sentries out regularly?
I would again argue that the 20% increased damage for every source of damage from something like steady aim would yield a greater return than getting a 5th sentry out once in awhile. But it's going to be pretty hard to quantify it. I also run physical so I'm seeing increased damage from pets and I run a hatred generator so I can contribute damage as well. Tomatoes tomatoes.
I greatly prefer vault for survivability. I wouldn't call smoke screen infinitely better. With smoke screen it's too easy to get hung up on walls and mobs and be unable to get away from the source of damage. Plus you can kite around sentries a lot easier.
Once 2.1 hits live, it might be a different story with the greater rift climb. In that case you'll eventually hit a point where you'll need all 5 sentries. Still it's a hard sell for me.