I see about a 10% performance improvement on my local machine on the input in test.py when not constructing the unnecessary second list.

I don't really buy that it reads nicer in prose since you can just do (a + b).sort() if you want. Plus, I feel like it's important for readability to not be unnecessarily redundant. Having list(a + b) code also risks creating misconceptions about how lists can be constructed and used in Python.