Locally, a friend and I have about 10k miles between us with 10 spd chains on
ll spd cassettes without any noticeable problems. Mybe we just aren't as
tuned to our bikes operation as much as some seem to be, but no dropped
chains, missed shifts with either mechanical or Di2 shifting. Which is great
as I picked up a dozen NIB KMC 10spd chains for $12 each 8-10 yrs ago,
probably a lifetime supply.