no but i mean will they be able to tell if i have or not?
I'm not 100% sure but I suspect there will be some dead giveaways in the diagnostics that will tell them that the phone hasn't been restored. And if they catch you lying, then the likelihood of having a successful resolution drops a bit.
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Does it feel like your 6+ is doing maybe 15% better then a 5 or 5S? Then it's probably performing appropriately.
It should be more like 20-40% -- probably more if you're judging against a partially degraded 5/5S battery.
I'm getting at least 20% more with the regular 6 over my 5 (probably much of that is degradation of the 5's battery)