I don't think hard evidence is going to be difficult to come by for some claimants. I imagine it would be fairly easy with a list of Apple ID's, registered iPhone serial numbers, and battery test records to construct a comprehensive list of people who had their phone battery tested by Apple, indicated passed, and then registered a new phone to their account shortly thereafter. That list would almost certainly be admissible as parties who may have suffered a loss, and just like the iBooks conspiracy case compensation could be automatically given to the identified parties without any action on their part.
There will be another category of claimants who had their battery tested, told it was fine, and then bought an Android phone. Again, it will be pretty easy to document the date of the battery test and the date and amount of the subsequent purchase to establish the potential loss once the identified group of consumers is notified.
I would also want the records generated during the iOS 10.2 test phase that showed the number of random shutdowns and after iOS 10.2.1 was released that showed what percentage of phones were throttled.
If I were an attorney that is where I would start, with a set of records provided by Apple that would lead me to identifying as many documentable claimants as possible. If it is high enough percentage of devices I can see Apple offering a settlement where any original purchaser of an iPhone 6 or 6S is entitled to compensation.
This is why you'd be eaten up by the lawyers.
Remember, what's at play is NOT that the alternative was 'random rebooting', but that 'People where not told'.
So, one of two things was going to happen to these old devices: 'Random Reboots' or 'Slowdowns'.
Now each litigant is going to have to prove that, given that an Apple battery at the time was priced at $99, how many of them, when faced with one of those two outcomes, would have ONLY brought a new phone if the 'slowdown' was a factor.
This is why this is a veritable minefield for the litigants - they have a huge hurdle to overcome - show that they purchased a new phone ONLY because the device was throttled, but they would NOT have done so had it been rebooting at 40% battery.
Good luck with that.
Even then, they'll not get the money they paid for the new device - that'll not make them whole - it'll make them more than whole. Making them whole will be giving them the cost of the battery cost - so, the chances are, for the litigants who make it through to the final round, a MAXIMUM of a $99 check (or, more likely, an Apple Gift card to the value thereof) awaits them.
This being the case, it leaves the 'People were not told' argument. And for that, Apple have the EULA as the Ace in the pocket. Bad PR, yes. Legal - very probably.