I see illusory armor as a defense set, making opponents think you're in one spot while you might be a little out of place, like D&D displacer beasts. I don't think they need to actually displace the player character model to do this, it would be more flavor text. I would make it mechanically similar to Ninjitsu for scrappers in that it would also have some mitigation effects, perhaps a confusion aura and a self heal that works like illusion control's spectral wounds that deals damage that fades after time. You get a big boost of health and healing and then it fades over time, which might be enough time for you to recover from injuries taken.