I've been cleaning out my phone and just now thinking I'll switch iCloud on.I agree. I still sync photos from a folder on my Mac and won't move to iCloud until if/when a few changes are made like the option to move images to a folder instead of a alias and the ability to export but retain the date created/modified.
After reading your concern I tried importing a pic to Photos, changing the exif date, then exporting. If I export as an unmodified original file, I can opt to use the title as the filename. As you pointed out, there's no way to have the original photo with the new exif date. However, when I export as a .jpg (the original file format for my example), I'm seeing any exactly 1byte difference in file size. I'm not sure if there's compression going on, but I'm really not seeing anything else indicating that the image itself has been altered.
Maybe someone else can weigh in. Since EXIFTool and many other tools can modify original file exif data, I don't get why Photos cannot.