Apple - How to export photos from the new Photos app in full quality?

File > Export > Export Unmodified Original...

will export a full size image to the folder of your choice.

I found the original in:

~/Pictures/Photos Library.photoslibrary/Masters/

I still need to find a way to tweak the awful default behavior...

The title of your question asks how to export photos in full quality, whereas your last sentence asks how to recover your original photo. @JaimeSantaCruz is absolutely correct that you can Export Unmodified Original, which will give you a full-quality copy of your original. However, if you've made edits to the photo, they will be lost with that command.

What I would suggest is using the standard Export command instead (⇧ shift ⌘ cmd E).

If you expand the Photos section by clicking the ∨ button you will be given the choice of several parameters for your export.

Make sure you pick Full Size as the Size.

For JPEG Quality, I recommend High for photos taken with an iPhone or iPad, as I believe this is the quality setting used by iOS to save photos.

As a test, I emailed an iPhone photo to myself as Actual Size, from my iPhone then from macOS Photos (using the Share action), and also exported it using Unmodified Original, High and Maximum.

Unmodified Original, High and email as Actual Size from both iPhone and macOS all came out to the same size (although emailing from macOS occasionally produced a slightly larger file). On the other hand, Maximum produced a file almost three times as large, with no perceivable difference in quality when blown up on a 27” non-Retina iMac.

Therefore, I would only use Maximum for images imported manually, known to use a lesser compression (higher quality) originally.

Note that when you Export an edited photo, Photos re-encodes that photo (necessary to apply the changes). That is why the size of the exported photo won’t match the original. Using Export with High quality on an unmodified photo should produce the same photo as Unmodified Original.

Personally, when I drag an iPhone photo from Photos to my desktop, it is exported with the same size and quality as an Unmodified Original or a photo shared by email as Actual Size. As far as I can tell, this is not a case of Photos simply applying your last-used settings. So I'm not sure why you're seeing different behavior.

It’s also worth mentioning that you’ll get to choose compression options when you export videos as well. However the videos appear to be re-encoded regardless of the settings you choose for Export. They do not get re-encoded when you use Export Unmodified Original.