Here’s some testing on the S7 with T-Mobile using 4x4: http://cellularinsights.com/samsung-galaxy-s7-the-first-4x4-...

There’s a perceptable impact to users with mimi and it specifically takes advantage of multipath. This is not a case of marketing fluff.

However, 256QAM is something you’ll probably only see in ideal situations as it requires very good signal, that’s the one that edged out gigabit speeds in the lab.