I too play ripped UHD movies, although I usually don't normally use Plex. I agree that the Shield is the better player for local ripped media, mostly because it supports bitstreaming the lossless audio to an AVR and supports image based captions (Roku does neither). But I can still play my UHD movies on my Roku (with the lossless audio transcoded to AC3) using WiFi instead of Ethernet.
I have tested my Ultra 4800 on both Ethernet and WiFi for max speed, as well as playing directly from the USB port to eliminate the network from the equation. What I have found is the Ultra will max out at roughly 220 Mbps on an 802.11ac access point that is less than three feet away from the Roku. Playing some fixed bitrate test videos, on Ethernet it starts having issues at 90 Mbps (completely expected on a Fast Ethernet connection) and can play reliably over WiFi up to about 200 Mbps (which matches the reported speed in the network test). But I also found that even using USB the Ultra can't go faster than 200 Mbps, so that's pretty much the hardware limit of the 480x Ultra. I have a 4850 coming, and I will be repeating these tests to see if the latest hardware is any faster.
My ripped UHD movies have a 60-70 Mbps average bitrate, but they all have peaks that can exceed 150 Mbps, so Fast Ethernet will never suffice for that content. But the Roku is perfectly capable of it with a good WiFi connection.