Definitely sounds like the cat material inside of them is broken, causing it to rattle loosely inside, nothing to do with the Invidia Gemini, more so to do with the age and use of the OEM cats.
It's a very common issue on 350z's, especially as they get older.
That being said the HFC don't cause much drone whatsoever, most are 200 cell vs the OEM 600 cell.