If plants do the bio-filtering, why do some suggest 'overfiltering'?

The big thing you will read about here on this site is flow or circulation. Really good flow is encouraged so good nutrients get to all of the plants ... it is also healthy for fish, as they get more exercise. Plants do block circulation quite a bit, so in a heavily planted tank, you go for more flow than is recommended for a simple fish tank.

 

In my situation, I have coldwater macro algae in my tank, and although I do more circulation than filtering, I do still skim, and remove particulate matter using an undersized canister filter (Eheim 2028). My canister has the wool in it, that's all.

As you no doubt know, detritus forms all over the place, and good circulation/filtration can help remove the floating bits...

RAS

 

Plants can use up only so much ammonia and nitrites. Yes, they are like bio-filters in a sense (and pretty good ones), but they are not so good as to render bio-filters useless. Since ammonia and nitrites are very toxic to fish and other critters, the bio-filter simply helps convert ammonia and nitrites to nitrates, which are much less toxic to fish.

The mechanical filtration is important too, of course.

I'm in about the same boat as you regarding the gph of my filter (260gph to 50G) compared to your XP3 and your tank size (350gph to 75G). I consider my filter underpowered (and want to upgrade to an XP3 actually). I make up for my current filter in 2 ways: 1) My CO2 is distributed via a 185gph powerhead, so I get a lot of extra flow during the day (you should see my plants swaying), and 2) I run a Marineland Emperor 280 gph bio-wheel filter all night long (on and off a few times per day to keep the wheel moist).

Honestly, I'd rather have a more powerful canister filter and remove the Emperor completely -- for aesthetics alone.