Another possibility, and one I would recommend even more especially if you are "pushing the limit" of your tank capacity, would be to put in a second biomedia chamber/insert. Because THAT is where 95% of your biological filtration is happening, so you will increase significantly your ability to keep ammonia & nitrite levels at zero. Plus, should the need ever arise to replace the biomedia, you can replace one of the two packets of it at a time and you don't have to worry about your tank having to re-cycle. Or, alternatively, should you ever get the itch to set up a second aquarium (not that such a thing EVER happens to people like us), if you buy the same filter for your second tank, then you can pop one of your two biomedia chambers into your new tank and BAM!, your 2nd tank is now instantly cycled.
And I also give a +1 to what meegosh said, I personally have found that a big chunk of filter floss does better at polishing the water than foam does. Foam is fine for removing really big chunks of stuff (plant leaves etc.) but when it comes to removing finer particles the filter floss does much better.
I don't think that leaving the activated carbon in your filter will *hurt* you at all, it just...well...doesn't really do anything positive for you unless you are needing to remove medicines from the tank or something like that. But under normal operating circumstances, if you have toxins in your water that your carbon is removing, then something is really really wrong with your tank!
(Actually, now that I think about it, it *does* hurt you if you have a tank with live plants and you ever dose plant fertilizers, as the carbon will think the fertilizers are "toxins" and remove them from your water.)