The "55 gallon" rating on the filters is likely not a rating for how you are stocking the tank, so running two of them is perfectly fine. If you are thinking of running a 3rd, 4th or even a 5th filter (taking it to an extreme), at that point you are better of taking all that money you would have spent on the filters and upgrading to a larger tank. Increasing water volume will stabilize water parameters much better than running 5 filters on a 55G.
I agree with you here. Redundancy and having the extra capacity to deal with an ammonia spike is definitely a good thing. I was taking the example to an extreme...I have seen sumps on here that were so packed to the brim with ceramic bio-rings that they would have been better off spending all that money wasted on that bio on upgrading to a larger tank. At a certain point, the money thrown at extremely overfiltering a tank would be better put to use by upgrading to a larger setup.
My point about adding a ton of bio-media is that there actually is a law of diminishing return when it comes to bio-media. When the bio-media ends up acting more like mechanical media, then you have too much bio-filtration.
I would love to have a drip system, but my filter serves it's purpose well. I am happy with it. Cleaning filter socks every week is a pain, but I am removing waste from the system. If I had 15 filter socks instead of 3, I could get away with cleaning them less often since they wouldn't clog as quickly, but my water parameteres would not be as good as they are because I wouldn't be removing waste as frequently. This is another example of less being more.
Admittedly, I rarely check my water parameters anymore either. I'm not promoting narrowing the band to the point where an extra feeding is going to result in ammonia readings. I'm taking the OPs question to mean is there a point of diminishing returns and in my opinion there is.
