Any issues with applying an extra coat to fully cured pattern coat hi gloss? I'm flatting to 400 grit and have a few break through's.

Should be fine, just make sure you prep the surface well. Not sure if the Pattern Coat range is thixotropic enough to avoid fisheyes, but they are a pain to rub out when they do happen so best to make sure they don't. Clean surface at a minimum, possibly very quick scuff with some 120 grit as well.