When you use extensions, they save JPEGs into the photo library. When you ask Photos to export, it will just use that JPEG. It does not go back to the original + invoke the extension.
When you are using RAW Power as a standalone app, it is still writing a JPEG into the photo library. However, if you use RAW Power to export, it will generate new images from the original RAW. So, you get full fidelity. That, of course, is only true for images edited in RAW Power. If you edit an image in Photos (or any other editor that uses the photo library) and then export that image in RAW Power, it will use the JPEG because it has no way to invoke Photos to render the image from the original.
If you plan to use multiple editors through the Photo library, then you are inevitably going to be stuck with a JPEG at some point. If you want the highest quality then use RAW Power to edit the image (standalone), export a TIFF and then edit that in the other app. I suppose you could import the TIFF into Photos and then use a separate extension to edit (but again, you have a JPEG at the end).