Nice work on creating this but, in my testing, this does not seem to work for me. In fact, the photos that Photos+ is downloading appear to be of a lower quality (at least according to the metrics). I've also noticed that Photos+ is stripping out a lot of useful JPEG metadata. Take a look at my outputs below.
# jhead command gets the JPEG metadata
$> jhead IMG_1234_orig.JPG > metadata_orig.txt
$> jhead IMG_1234_better.JPG > metadata_better.txt
$> diff -y metadata_orig.txt metadata_better.txt
File name : IMG_1234_orig.JPG | File name : IMG_1234_better.JPG
File size : 7546221 bytes | File size : 2967754 bytes
File date : 2019:01:01 01:23:45 | File date : 2019:01:01 01:23:45
Camera make : Canon <
Camera model : Canon EOS 760D <
Date/Time : 2019:01:01 01:23:45 <
Resolution : 6000 x 4000 Resolution : 6000 x 4000
Flash used : No | JPEG Quality : 90
Focal length : 18.0mm (35mm equivalent: 29mm) <
CCD width : 22.68mm <
Exposure time: 0.0020 s (1/500) <
Aperture : f/8.0 <
ISO equiv. : 100 <
Whitebalance : Auto <
Metering Mode: pattern <
Exposure : aperture priority (semi-auto) <
GPS Latitude : <redacted> <
GPS Longitude: <redacted> <
JPEG Quality : 97 <
Likewise, the filesize is much smaller in the "better" version.
$> du -h IMG_1234_orig.JPG IMG_1234_better.JPG
7.2M IMG_1234_orig.JPG
2.9M IMG_1234_better.JPG
Perhaps something has since changed in the way Google Photos handles downloads? Apologies - I did not get to try this extension when it was first released, so I have no point of comparison.