That is certainly a solution and I considered it, but then I struggled to push the wheel into the dropout without an extra washer and I decided against it.
The washer is thin, and I will just have see how it goes. I do have a question for you. You stated in another thread that Bikegang was supplying 11-speed cogs. How do you know? For that matter, given a box of cogs, from different sets, how can you tell which speed set a given cog is from? I assume by measuring the thickness, but where? The tooth or body?