I admit I've never really understood overstocking. I am aware it can work well with certain fish such as African cichlids so I guess this doesn't really apply to them but I think a lot of people have a misconception about overstocking creating additional activity and busyness.
From purely a visual perspective is this really what you are seeking? An overabundance of fish, IMHO, only creates one thing: clutter. Movements become less synchronized and more chaotic, certain graceful fish begin abandoning natural movements and pathways, and a lack of negative space leaves little for the imagination to fill in. What I strive for in my tanks is an environment (to the best extent an aquarium can take it...) that can create a harmony where the fish balance with the decor and negative space and allow for moments of calm and moments of bursts of activity. This allows for greater appreciation of the finer details in our tanks, similar to the shards of silence between notes in music or the emptiness that surrounds a particularly meaningful painting on a wall. I guess my point is is sometimes it's less about how many more fish can I put in and more about how much space am I giving up?
With that said I do want to stress that that's just an opinion and as long as your fish are healthy you can do whatever you want.
However, I will say that overstocking is typically not recommended because the water chemistry aspect (i.e. overfiltering) is only one small thing in how it affects your tank, as it can lead tremendously to added stress (extra aggression, not enough space to move around, territories to claim, or space to hide), certain fish being out competed for food, and so forth. I would never say that simply adding extra filtration or keeping up with the water chemistry is enough as it all depends on the type of fish, size of the tank, and a million other variables too. All of these things can lead to shorter lives for our little friends, so just some things to consider.
