Does anyone know the weight of the buffer that comes in the ATI kit? I have a middy and want to make sure that I will not have short stroke issue. Thanks for your feedback.

it is carbine weight.

Will the standard carbine buffer be heavy enough for a 16" barrel middy? Or should I go heavier?

There is no way you can get this answered via the Internet.
Carbine weight buffers will work in just about anything and are the least expensive, that is why so many ARs ship with them.

To get the perfect weight for our rig and the ammo we choose to run, some of us carry all weights in our range kit.
You swap them out until you find the one that runs best.

No way to preempt that via the Internet.....you will just be getting guesses from strangers.
But hey, it's free advice !

my advice would be to pick up a better spring as well as a heavier buffer.

I helped my friend build a lower in 2011 and he went with the ati kit.  the spring crapped out on him early into his second case of ammo.

without knowing who made the barrel, the gas port diameter and what ammo you shoot all i can give you is a very poor guess.