Is there any benefit to using AMP now?
AMP's biggest performance improvement is its ability to lazy load images and ads. From https://github.com/ampproject/amphtml/blob/master/spec/amp-html-format.md:
- HTTP requests necessary to render and fully layout the document should be minimized.
- Resources such as images or ads should only be downloaded if they are likely to be seen by the user.
- Browsers should be able to calculate the space needed by every resource on the page without fetching that resource.
If your page has images or ads below the fold, it is likely that AMP will be able to render your page faster by lazy loading the images.
AMP places restrictions on development. It limits which HTML features you can use. AMP may or may not be straightforward to implement because of these restrictions.
AMP may be able to optimize your site beyond the 100/100 score on PageSpeed Insights, however it is not straightforward just to tack in on, and if you don't need image lazy load, AMP itself can be added weight to your page.