I've been thinking about this issue for a couple weeks now. I think like all things, the answer is..."it depends" I struggle with the mental prep part of competition a lot and for many years seemed to only hit big goals when I was least prepared and least expected it . I came into some my best bike races completely hung-over and shitty. My mental prep a was that I was pissed off and had nothing to lose.
Now, I do most of my training at a PL gym where all top sets are done with some degree of rage psych up as mental prep.. I find that most of the time, it actually is detrimental to the session...I get too angry and lose focus, or get too aggressive with the weight selection. Even if it let's me hit PR's every other week, the danger is that sessions will devolve to record poaching.
OTOH, I've had some amazing competition experiences where I went into the event relaxed, gradually worked my focused until I was completely jacked up and then nailed it. Doing this takes a lot of practice at turning it on and turning it off.
Here's what I try to do,
At all times be mindful of what you're doing and why you are there. If it's a volume day, getting you're Gorgoroth face on is not going to help. Focusing on perfect movement, staying at lighter weights if need be and really visualizing your practice between sets is the important mental prep.
OTOH, if it's a PR day or a competition, you've got to let yourself go a little at the right moment but stay relaxed until it's TIME. For me, it's ok to come a little unhinged. Just like with a true max effort lift, things look might get a little ugly or borderline, The mental state can be a little sharp and raggedy. It's OK to care enough about the outcome to get excited.
The key I think is to learn how to get yourself in and out of that state quickly without letting it rule every session. Use it as a tool.