I would increase your filteration by adding another filter (a canstier would be nice), filter that come in package deals often are inadequate for the tank they are sold with. I would also change your water changes to once 40-50% a week and fast your fish on that day (if you don't allready).
I might buy the EXTRA large ghost shrimp at my LFS. They're the size of my entire fist with some of my wrist included. It's crazy!
My current setup is the Aqueon 55g package. Aqueon 55g filter, I do 10g water changes every 3 days.
It's an Aqueon Power Filter 55. It's the ones you buy off the shelf. I think it can adequately keep up.
Right now in my 55G I have...
1x Pictus Catfish
1x Rainbow Shark ( Getting rid of soon because of aggressiveness. )
7x Tiger Barb
2x Green Emerald Cory
2x Ghost Shrimp
Is this overstocked? Keep in mind the shark is going soon.