I'm using a BitMain APW5 PSU for each one which are rated at 2600 W each. So I set it to 2600 W since I am running on 240V. If this is incorrect, what should I be setting it to?
They were both bought in May, 2017. One is a 13.5TH and the other a 11.5TH.
I think you should leave some margin just in case. 2600W is a crazy lot for an S9, if you had that with immersion, you could even reach 20TH (if the chips are in good condition). I know this because i already know of such a case with a powerlimit setting of 1675W and its at 19TH already with the chips at 49°C, so it was only hold back by the PSU. Measure at the wall just in case, and try not to get close to 100% capacity. An S9 is unlikely to ever demand 2600W anyhow so i doubt it would be a problem, unlike the 1600 and 1800 PSUs who can be put to the max with a single unit.
I agree with faheshang about the factors for speedier autotune, i have observed the same thing with others. Hopefully there will be some sort of "saving state" of the autotune in a coming update so it doesn't have to do it all over again everytime you powercycle the unit or change some unrelated setting like the pool.
Of course the powerlimit also indirectly affects the temperature and speeds the autotune will try, if you use a lower setting, it results in a colder slower unit (as efficient as it can get within the allowed powerlimit).