Nope, definitely not overkill with a messy fish like an Oscar. You really have to work hard to keep an oscar tank clean to avoid stuff like Hole in the Head disease. I'd try to keep nitrates below 20ppm and do water changes immediately when it reaches 20ppm.
My 75 gallon tank that used to have my oscar (which I had to euthanize due to hole in the head) had 2 emperor 400's and one Emperor 280 on it). I still don't think it was enough filtration. Definitely use your gravel vac every time you do a water change and do LARGE water changes. At least 50% per week, twice a week would be better.