It depends on what you're trying to achieve here really and how you're measuring the results.
It sounds to me like your theory could have merit. But are you concentrating your efforts on trying to achieve a more consistent muzzle velocity or are you trying to trying to achieve downrange repeatability (accuracy) ?
Because one doesn't necessarily lead to the other. What you may end up with is deeper scoring from the rifling on your pellet skirt than you otherwise would have had without 'upsizing' your skirt and that may lead to reduced repeatability downrange.
Forget about measuring muzzle velocity, instead you should measure downrange velocity to check your results. If you find greater downrange consistency, then move on to testing grouping sizes.
When people say its a waste of time, what they mean is that the time you expend on this testing could be much better spent - if your ultimate goal is to knock down more targets in FT or HFT. I would agree 100% with that opinion.
If you want to knock down more knock over targets, find a die/batch of pellets that group well straight from the tin buy lots of them and then shoot as many competitions as you can. When you miss a target make a mental note of why you think you missed and then use your time to practice not missing for the same reason again.
Ask yourself how many World championships the guy in the YouTube video has won?
Personally, I'd rather take advise from someone who can walk the walk.
One of the best things about this sport (both FT and HFT) is that the people who are mega at it will go out of their way to give you truly helpful advise. Coner (former world champion) says he uses pellets straight from the tin, Cozy (current World champ) - straight from the tin, Ian Taylor (3 times world champ) - straight from the tin, Calps (he's won everything at one point or another) - straight from the tin, James Osbourne (former world champ) - straight from the tin.
I could go on and on - but the pattern should be clear