Depends on the part, there are 2 at work here (from the original mk3)
The first one is the angled fan. The stock fan you get is smaller, but I wanted a quieter fan and this improves cooling by 20% (according to the author). But if you replace the stock fan spot, it will hit the Z axis when it homes. With this mod i can have the bigger fan which cools it down faster (the hotend when done printing) and is quieter when runninghttps://www.thingiverse.com/thing:3484443
The second is this part, I customized it and bent it so I could fit all threehttps://www.thingiverse.com/thing:2978427
This improves cooling for the nozzle itself, helps with bridging and supports and whatnot. Example pic here https://cdn.thingiverse.com/renders/ee/73/f7/64/cc/f0b72f75684a01b74ff0f459a9f7b898_preview_featured.jpg
The first mod helps a tad, but not so much where you would see a difference probably. But the second mod (with the 2 duct fans) does help parts print better (at least overhangs), you would probably see more of a difference in that one if anything.