I’ve got the Garmin OS 1:50k map on an SD card, but it’s really quite slow to scroll on the Edge 800 it came with, probably the SD card’s fault but it’s protected so you can’t clone it to a newer faster one.
I downloaded the OSM cycle mapping for the UK onto a new 64gb U3 class SD card, had to wipe the card, partition it to <32gb and format as FAT32 to make it compatible and it’s much faster, although the difference between FP and BP/BW/Road isn’t as clear.
But now I’ve got a problem with the routing, the Edge doesn’t have enough memory to even get me somewhere 20 miles away! Presumably, because the OS map was only ever routable on roads, now when faced with probably 5x more tracks and exponentially more routes to check it just crashes!