I’m seconding Craig’s vote for NeatVideo. Have used that many times to remove noise (from severe to hardly noticeable). Worth every penny.
FilmStyles filters are awesome for color grading.
“Mask 8 points” by Alex4D makes masking a lot easier. (It’s an 8 point mask up from FCP X’s 4).
DaVinci Resolve Lite (not a plugin, but a great free third-party tool).
7toX if you’re going from FCP 7 to X.