WiFi coverage and speed is a really complicated topic. The dual band "10" series Orbi's use the one 5G WiFi channel for both backhaul between router and satellites and connection to use devices. Every Speedtest data packet consumes radio time going from the router to satellite, then more radio time going from satellite to phone. Anything happening with the other two satellites (even "beacon frames" and other management overhead) also consumes radio time.
That 866mbs speed described in the product literature only applies when two devices are (literally) right next to each other. WiFi performance falls off with distance and obstacles, such as walls and floors. So, yes, in one sense having fewer satellites can improve performance. 5G WiFi is much more impacted by distance and obstacles than 2.4G.
Dimensions are also important. 1,600 sq. ft. could be a 40x40 square, or an 80x20 rectangle. The square would probably do fine with a single satellite in the center. The rectangle might be better off with two. Three definitely seems overkill.
The 450mb wired baseline is useful, but another baseline would be to test the phone upstairs close to the router. That is the absolute maximum the phone will ever achieve. Compare satellite results against that benchmark.
The "10" series product was designed to compete head to head with other dual band systems where cost is an important consideration. When customers have 100mbs internet connections, that 5G bandwidth is probably more capacity than the customer can generate.
Reading this, I do not know if it adds anything you did not already know. (a) reduce the number of satellites, and (b) shorten the distances and obstacles as much as you can while covering the entire house with a good signal.
