I came across an interesting article/blog thing while researching the compatibility of shrimp a while back. I was trying to make sure the species I was adding didn't interbreed and create hybrids. Anyways from memory, the article argues that while a shrimps bio load can be minuscule, it depends on what you're feeding them. If you allow your shrimp (in a planted tank) to do their jobs and just scavenge/feed of algae and other naturally occurring food such as plant break down etc their bio load remains tiny.
However, if you "feed" your shrimps (pellets/fish food etc) their bio load can have an impact.