Comparing the situation with the Moto Z to the iPhone situation is very much "Apples" to oranges. (Haha see what I did there)
The iPhone issue was first off, intentional, and second, affected every single iPhone sold since the iPhone 4s I believe was what's been stated. That's hundreds of millions of units worldwide, almost 30% of the entire smart phone market, and growing as Apple has said straight out they have no intention of discontinuing the throttling of older units (going forward, they will be giving a toggle buried in the settings to turn this off for those that wish to)
I haven't looked at any sales figures but the market share of the Moto Z is a mere sliver of a fraction of the market. (Motorola was hoping for 1% for this phone when they launched the Moto Z) And then we look at a fraction of those Moto Z owners that are affected, I bet we wouldn't even hit 20,000 units worldwide. Unfortunately unless we could find every single one of us on the planet affected to sign up, we are out of luck. And then even if we did, it would cost us much more than a replacement phone to pursue it.
Another issue is this. Unlike the iPhone, the Moto Z is one model of several that Motorola sells. I haven't found anything closer to this coming of complaints with other models or variants (there are a few out there, yes, but nothing like the moto Z users) And they have no idea why this issue is happening in this model (with all too many representatives denying it's an issue in the first place)... So any suit would need to prove it was Motorola's fault in a court of law. For the Apple case, they came out and said "Yes, we are throttling your phone's performance the older they get. Your welcome. Yes it's an easy fix just change the battery. That's right, it can only be done through us and we charge such and such an amount to undo what we purposely did to your phone, simply by replacing your battery. No we didn't tell you this would happen.. sorry about that. But we can't understand why you care, you should all just be happy you have an iPhone" (cough cough). The proof part was all done for them.
No I think our options remain limited. Send it in under warranty. For those out of warranty pay the $150 and get it repaired (there's no money for Motorola in that, it pays for the components, shipping, technician's fees, and the shop which is third party), or you can switch manufacturers. The phone is two years old and quickly approaching end of life. Many of you are coming up to renewals soon. Don't get a Moto this time.