In Wi-Fi networking this is most certainly the case and to be honest I would expect a similar thing to happen with powerline networking. An 802.11b (11 Mbps) device connecting to a Wi-Fi router will cause any 802.11g (54Mbps) devices to fall back to 11 Mbps so that they can all communicate properly with each other irrespective of whether they are passing through a router device.
While it is typical for networks to operate at the slowest components speed it is not always the case, network switch devices can handle ports being at different speeds as the data is buffered prior to going out on another port and the segments are effectively isolated from each other (not the case with Wi-Fi or powerline Ethernet) and so it is not necessary for each network device to operate using the same line speed or protocol.
I would very much expect your 85 Mbps devices to fall back to 10 Mbps in a mixed network, but it could be that they are intelligent enough to work well together.
One way to test would be to do some large file transfers between the 85 Mbps connected machines (like video files or something) using Teracopy as it has a slightly better speed readout than Windows (iirc). Test it both with and without any of the 10 MBps devices on the "network" to see if there is any real difference. Also testing the speed to/from the 10 Mbps devices would be worthwhile.