I think I've had similar experiences (although not 100% sure) - making a concrete tiled floor, using a single concrete map (see attached).
I couldn't get anything to work with Real-world texture sizes - so ended up using with the tiling of the initial bitmap (I think ending on something like 0.15 as the map was huge). Then used the UVW randomiser to add random offsets and rotation. Finally plugged that into the Corona MultiMap to randomise it across the tiles/planks.
I'm sure someone very technical will point out some issues with this method, but it did the job.