Funny, basicly we are speaking here about mounts and not about the telescopes although the threadtitle suggests otherwise,...just saying.
There is no way a 5" SCT performs better than a 8" SCT as Eddgie explains as well.
You clearly didn't watch the movie. Even the Youtube poster acknowledges that the 8" will outperform the 5" optically. The valid part of the poster's video lies with the mount. Not only is the 5" lighter, and likely to be used more, the 8" is a bit too large for the mount. People will get a more satisfactory experience with the 8" on a higher capacity mount. Don't underestimate the mount when it comes to telescope performance and satisfaction.
The beefier mount also contributes to overall portability and ease of set-up issues. It's not just SCTs that have this issue, all designs do. People love 6" ED refractors and gush about them, but a year later they either don't use them as much, or they are thinking of selling them. Dob owners decide that maybe they bit off more than they could chew, or advancing age will allow, and downsize in aperture and f-ratio.
The advantage of the Nexstar 5 is that it is still a "one arm carry out the door" scope that performs pretty well. The Nexstar 8 isn't, and the Evo 8 certainly isn't.