Hey man
I haven't a direct experience on e-commerce AMP, but I know something on that, so just one or two considerations:
1) Zalando and AliExpress are using AMP (or testing it at least), and AE said that they saw an important growth in conversion rate.
2) At this moment, you still can't create a dynamic cart with AMP, if I remember well. That means users can't change how many items are in their cart without restarting the checkout. In general, you can't create some of typical dynamic e-commerce's things that can help users and enhance your CvR, so I think that if you are planning to test AMP on a little e-commerce, that can really help your sales but, if in the past you have seen that special effects had an important impact on CvR, I wouldn't go with it.
About rankings: I think that, and I've seen it working on blogs and online newspapers, AMP alone can't be a ranking factor, but UX is. And AMP can have a big impact on UX.