I don't doubt you saw an increase in speed. I'm just not sure the increase is for the reasons you cited. The increase in speed is more likely because you had one radio dedicated to your Mac, so you didn't have to share that radio's bandwidth and time with other clients. Wireless bandwidth is a shared resource. The more clients connected to a radio and the more they are using data the less bandwidth available per client -- even if all the clients are 3x3, 80MHz, etc. The wireless radio only talks to one client at a time, so that naturally creates a slowdown directly related to the number of clients that are using that radio, as the radio alternates transmitting data to each client one at a time.
Oh I get that. What I'm saying is that my link speed without my iPhone on the same channel was 1300 and with the iPhone on the same channel was 867. Taking the phone off immediately increased my link speed. And it's not just empirical - see this article, which says, in part:
While most 802.11n and 802.11ac clients support 40 MHz channels, not all 802.11ac clients support 80MHz channels, and no 802.11n clients can support it. So when an 802.11n client connects to an 802.11ac radio using 80 MHz channels, what happens? The radio drops down to a 40 MHz channel or 20MHz for that connection depending on the capabilities of the client. Obviously this means that during all communication with non-80 MHz clients, a 60 MHz or 40 MHz slice of channel capacity is going to waste.
After reading that, I tried it by taking my iPhone off the channel. (Frankly, I'm amazed I was able to find that article again.)