Thanks, Pablo, for providing that. So the changes look like mostly a wash on these benchmarks. 

Serhiy:

I do not see any significant change in + operator timing on my machine (again, just a rough test):

$ ./python.exe -m timeit -s z=0 z+0  # master
10000000 loops, best of 5: 21.3 nsec per loop

$ ./python.exe -m timeit -s z=0 z+0  # list-add
10000000 loops, best of 5: 21.2 nsec per loop

I'm aware that unpacking is "better". With that said, adding a list literal (or a slice of any list, or a list comprehension) to another list is a fairly common operation (I count several dozen examples in the stdlib). Even though these cases only have two operands, they will still see the speed-up.

And the speed-up is good, even in these cases. You can compare using the new code:

$ ./python.exe -m timeit -s l=[0,1,2,3] [0,1,2,3]+l  # Hits new branch
5000000 loops, best of 5: 87.9 nsec per loop

$ ./python.exe -m timeit -s l=[0,1,2,3] l+[0,1,2,3]  # Hits old branch
5000000 loops, best of 5: 92.5 nsec per loop