There are several possible ways to quantify expressiveness of a basis. For instance, you can count how many unique lambda normal forms you can generate with com

There are several possible ways to quantify expressiveness of a basis. For instance, you can count how many unique lambda normal forms you can generate with combinator expressions up to a certain size (modulo some terms that take too long to normalize). I once counted this for various choice of single-point combinator bases and up to size 16, and found that the above S' produced over 2 million normal forms whereas for instance the more well known \x.x S K produced only 244 normal forms. When you look only at how easy you can recover S and K, then the latter is much superior though (length 16/11 versus 3/3).