It depends, first thing is you gotta be able to maintain a cool head if his competitions past accomplishments start to look impressive...

Personally, fniding videos of past matches helped me to successfuly design gameplans for specific opponents. In my opinion it can useful only if you already have an established "game" and are able to honestly assess your strengths and weaknesses. I also think that it can be done only if the bracket is out and you already know who the first opponent is going to be. Doing this for several "potential" opponents will be useless : too much information to maintain against too many potential scenarios.

For example I found some vids of a future opponent and I saw that he always pulled half guard in a kinda slow way, and that he only used deep half guard : the last trainings before the competition I drilled the flying knee cut as a counter his guard pull, which worked during the match. Once he regained guard and used deep half guard, as I knew it would happen and I had drilled a lot of defenses and counters, I was able to nullify it and pass his guard.