for example, you often hear vegans have more overall fitness and more energy
You hear it primarily from vegans who are into evangelization for their lifestyle. Not exactly an objective source. Color me cynical. But your personal experience with veganism could prove me wrong. Some people do well on vegan diets, some people need meat.
When I switched back to an omnivore diet — and my reasons were personal, not fitness based — I didn't notice any real change in performance. And I went straight for the steak, too. Dietary changes that have improved my performance have been independent of the vegetarian/omnivore choice; things like eating more and a greater variety of vegetables, eating less processed food, getting a handle on my crippling addiction to sweets. The kinds of things that tend to be universally effective.
I also dabbled with vegetarianism back in the 1980s, after reading the book Diet For a Small Planet
, and getting into a relationship with a woman who was a heavy duty vegetarian. We went our separate ways, and my commitment to vegetarianism lapsed. The importance of eating fruits and veggies remained - but you don't have to give up animal protein to eat a lot of fruits and veggies.
I was all ready to go vegan
Then I remebered bacon was from Pigs.
And Pigs are animals.