I have always heard that once grass carp get a certain size "they should be removed because their consumption of plants slows down". Why remove them? What are they hurting? Even if they just eat a small percent of what they did when they were young, they are still eating that small percent.
Bing, it's all about biomass and carrying capacity. If you remove a large one that isn't eating much (think about how much elderly people eat compared to a teenager), you can add more little ones, and more plants will be consumed.
I've caught them on Stubby Steves:
http://www.stubbysteve.com/