I think you are worrying about the wrong issues with printing banners.
Resolution and viewing distance are more important than color bit depth.
While 300 ppi (pixels per inch) is usually considered the standard for photo printing, you can get by with much lower resolution depending on the viewing distance.
Your banner 1260 mm x 2120 mm = approx. 4 x 7 feet. Banners and other large items are meant to be viewed from a distance.
How far away will this banner be from the averge viewer. 3 meters, 7 meters,10 meters or more?
At this distance, 300 ppi are overkill and will just bloat the file size and increase printing times. You can get away with much lower resolution, because at a viewing distance of several meters, our eyes are unable to see the difference between 300 ppi or for example, 150 ppi. Ask your printer what they recommend.
Remember you are creating a large banner that will be viewed from some distance, not a fine art print that will be framed, hung on the wall and viewed for less than a meter away.
Color bit depth only matters if there is visible banding in gradients or other parts of an image. This applies to only a small percentage of images, and are unlikely to be visible when viewed at a banner distance. While 16 bit printing is slowly entering the mainstream, Some printers (the hardware) and printer drivers simply don't work with 16 bit even if claim to do so. Most printers. (the people) will be expecting 8 bit files and may even convert any 16 bit files to 8 bit due to their workflow/equpment.
I would agree with Bo about the image size. My printer, which isn't very good, chokes at 300dpi. I print all my large posters (30"X40") at around 150 dpi, but depending upon the size, it could go lower.