Please add images for the said problem so we can tell what is wrong.
Sharpening should always be the last step in post processing. If you think that sharpening in photoshop deteriorates your image, then you should know that applying sharpening in VFB and then doing post process elswhere, would be much worse.
Nevertheless, it still would be nice to have basic sharpening controls in Corona, for those cases where external post processing is not needed.
An image would be really helpful, because I also have problem imagining this situation. 