How much rock to maintain biofilter

[flanque]

Hello everyone,

Here's a shot of my current tank. It's at an angle so it's warping the real front on view, but you should get the idea:

My 55G Tank Side View - Aquarium Advice Gallery

What I would like to know is how much rock do I need to keep in my tank to maintain a steady bio filter? I would like to "simplify" my tank and make it more open because the way it is now is starting to feel a bit cramped to me. Although I do not want to go this far, something like the following seems cleaner and more simplified:

http://www.seas.upenn.edu/~pcozzi/fish/tank.jpg

I currently have 55G tank with about 45KG of live rock. I have a 1200L/hr canister filter and a HOB multi-skimmer (skimmer and filter) which is rated to do a tank of 200L sufficiently (I have my doubts about this but that's another topic).

It's worth noting that I am a bit of a lazy tank keeper recently (kids) and would like to step that up a bit but it's worth considering in order to keep my tank maintainable and safe.

I assume more rock means greater bio filtering and generally a more stable environment but I also assume there's a point of taking things too far, so I am just trying to balance this out.

To be honest I'd like to see more open sand areas too.