The only downside I'm seeing to a closed loop is more equipment in the tank... But that doesn't really bother me. My tank used to have no equipment-everything was inline. It was a pita. It's not like anybody came over and said " wow there is no equipment in the tank"
Since then I have sold my lily pipes(which cleaning them makes them not worth having) I also sold my atomizer(worthless) and I still have an almost new hydor 300w chillin on a shelf. I'm now using the stock fluval input and out put with one of those tiny little terra heaters they sell at Walmart(I think it's a 25w) and I am 10x happier with it. I honestly don't know why that heater works so well in my 72g, but it does and I recommend it. It's small and easy to hide, it works great.
My tank is just starting to grow in again after a major rescape and equipment upgrade(replaced xp2 with an fx5)
I have the fluval running full bore.this tank will be heavily planted much like it was before I'm currently getting 600gph with my set up. I'm not sure that a flow reduction would really hurt me....but since my huge bga break out due to lack of flow I have become a flow nazi.
before the tear down
These are how it was planted before. I plan I plan on planting it heavier than before
this is now(lots of room to grow)