yes and no. You cannot open the original image in the Photos app. But that’s completely unreleated to the Preview Generator app. The Photos app just dosen’t do that. It always shows a compressed preview of the original file. But if you remove the max_size preview parameters or set them to a significantly higher value, you can open a full size preview or at least a nearly full size preview. (I don’t know if there is a hardcoded max_size)
And of course you can still use the Preview Generator app, if you remove the max_preview setting or set it to a higher resolution. The Preview Generator will then also pre-generate the higher resolution previews, which means that when you open images in the Photos app, they will be displayed quicker too, just like the smaller thumbnails.
Disadvantage of using the Preview Gnerator app like this: The previews need more storage space. because the higher resolution previews are generatetd for every picture, also for those you may never look at in the app. But it’s still a lot less than the actual file size, depending on size and compression of the source material of course.