I am happy with the stock mufflers, they have a nice bass sound to them without being loud, I have had the loud pipes and I have to say I have grown tired of them.
Reading the comment section of that video it sounds like he drilled out the catalytic converters from the mufflers. Not sure that is a real good idea for a couple of reasons, 1. It will void the emissions warranty. 2. The engine may run leaner which is not good. These engine computers can compensate some but they are limited and if the loss of back pressure is to much it could end up having long term reliability issues if the engine does run leaner.
Yamaha pretty much nailed it on this engine and I am hesitant to mess with success. While I did the stage one upgrades to both my 2013 Victory and 2016 Harley Davidson this Yamaha in stock form out preforms both those bikes with their stage one set ups and both of those bikes ran good.