Yes, there are many studies showing that the efficiency drops with smaller cogs.
But there are other factors like dirt that have also a major effect on chain transmission efficiency.
The same apply for wear: smaller cogs wear faster but also for wear, dirt has a bigger effect and it is well known that encapsulated chains last much much longer than chains exposed to dirt.
Now what does this mean in real world experience: as said for wear, I do not know yet. But for efficiency, honestly, I do not feel any difference between 9t or 10t and the classic 11t smallest cog (I also have two Moulton using the proprietary 10-28 Moulton cassette).