It depends on what you're connecting to. If it's all local, it can have a definite impact. If you're talking Internet related, probably not.
I also know you asked earlier about mesh networks. I will say that moving to my current gear increased my wireless coverage, which, I'm assuming, is due to beamforming. I really don't need the extender now.
It depends on if you have a wireless router capable of Wifi AC. If you do, it can make a huge difference on file transfers.
I transfer several large files and also do a Time Machine backup to my Mac Mini server that's hardwired with a 1GB connection. My MacBook Pro is running on Wifi AC and it was a noticeable difference between it and my older MacBook Pro that is not AC.
When I upgraded to a Time Capsule AC my Speedtest speeds upstairs went from ~50 Mbps to ~175 Mbps (iPad Pro) and ~150 Mbps (Apple TV 4). Don't have any numbers for a 2016 Macbook Pro, or what kind of throughput you can get from a Comcast X3 box.
Before committing I would try to find a device (phone, tablet, whatever) that supports AC and see what kind of results that you get in the location where you are going to use the MacBook. That's a lot of money wasted if the X3 doesn't deliver the bandwidth you want at that location.
It can do. As much as anything newer WiFi cards can be more powerful. With my 5th gen AEBS, I used to have to contend with neighbours' overpowering my router signals. Now with 802.11ac on both router and MBP theirs have taken the proverbial hike.