I've been wondering this myself and have also been doing tests on the 6s. I'm currently at the stage of recording how long the battery lasts without low power mode on at all. I'm only on my 2nd cycle since it only came out a few days ago.
Once I have a few data points I'll take an average, and then repeat the experiment with low power mode on permanently to see what sort of effect it has.
The thing is the 6s is fast enough to have low power mode on permanently with not much of a difference in user experience. In terms of benchmarks, it basically turns it into an iPhone 6 with a much better battery life - I'm just in the process of finding out how much extra juice it gives you.
I'll create a thread for it when I'm done, and link it in here for you.