What is the maximum WiFi speed of iPhone 6s? I have a new broadband plan of 1000 Mbps been effective for a few days. With a new ac router claiming a max to 1300 Mbps, I have conducted a brief test with my SE. It was at 267 Mbps, far lower than I expect. I am going to test it with an iPhone 6s which may benefit with MIMO. What is the top WiFi speed that iPhone 6s supported?
All iPhone 6s models support 802.11a/b/g/n and ac Wi‑Fi with multiple input multiple output (MIMO).

The iPhone SE also supports 802.11a/b/g/n and ac Wi‑Fi (but without MIMO).

While you should get better speeds with the iPhone 6s, the reality is that there are many factors that can impact speed at any given point of time.

However, if my reading of your question is correct, I think it's based on a misunderstanding of what the ac router claiming a max speed of 1300 Mbps actually means. This 1.3Gbps (1300Mbps) figure can be highly misleading because it is a theoretical maximum. It translates to 166 megabytes per second (MBps) or about 1331 megabits per second (Mbps), which of course sounds fantastic, but these figures are never realised in real world scenarios.

Most 802.11ac routers will perform closer to a range of 250-300Mbit. 

In terms of your devices, you'll find that the speed obtained by 802.11ac capable devices will differ considerably depending on how many antennas they can physically fit in. While the maximum supported by the 802.11ac standard is 8 antennas, most smartphones only have 1 antenna, most tablets will have between 2-4 antennas, and so on. Even 802.11ac routers will differ in terms of how many antennas they will incorporate.