Thank you very much for watching and for asking the questions!
My thinking is that the Zalman's omni-directional low restriction fin design gets hot air away from the cooler faster than any of the other designs. Extruded heatsinks have poor airflow obviously, and the bi-directional fins are taller and only allow air to travel in two directions. This is my hypothesis...I'm not a physics major or a thermodynamics engineer.
No mods are needed to put the Scythe fan on top. Please note that you need the 1900 to 2000 RPM scythe fans for this to work. The lower RPM versions are useless in this setup, and that is one of the reasons why I say the Noctua is an all around better choice:
1. Noctua is quieter than the Zalman by itself (though both are very quiet)
2. Noctua is easier to install
3. Noctua has good thermal paste, Zalman does not
4. Noctua's fan rigidly mounts to heatsink, Zalman clips in and can pop out in shipping
I have no idea why anyone would change out the fan on the NH-L9i. What else is there in the 80x15 range? I could see this if the Noctua didn't include a fan, but you would be throwing away money. The only thing I would replace it with would be a 25mm thick Noctua fan if you were putting it in a different PC...but then I would buy a bigger Noctua!
Drawbacks to the Noctua is the price, but that is about it...some people hate the color scheme. I admit it is far off from matching anything reasonably, but their coffee and creme color scheme I associate with premium quality so it has grown on me. I respect them for going against the grain and doing it well.
The theme of the video I was aiming for is that there is no such thing as "best" but rather there are choices we all need to make to fine tune--we can, however, narrow the range.
I've a Id-cooling Is-vc45 on order, and maybe that will be the best...it certainly is priced like it is. For now, I will continue recommending the Noctua and Zalman...but really I'm going to recommend the Noctua, because it is time consuming answering the question "which one should I choose?" every time.