@starrysunsets I'm not a nutritionist or dietician (so grains of salt), but the impact of this sort of diet seems to depend a lot on what you are doing currently. If you eat a healthy diet that balances macros, you may not notice much change or you might even feel like you are losing something because you are disrupting a successful diet in favor of hitting a certain number.
Conversely, if you are someone who eats junk and snacks throughout the day, this diet will no doubt shift you in a positive direction. If you swap out an Italian sub and bag of chips for with a leafy salad and an apple for lunch, you are absolutely being healthier and will see benefits. If you grab a handful of grapes instead of a sleeve of Oreos, that's a no brainer.
So, anecdotal reports are really only half of the story and may not apply to your situation at all.
For my part, I have never done the 800g challenge, but I have gone long stretches with the goal of eating 5 servings of fruits / vegetables each day and I would consider a hand fruit (apple, banana, orange) as a serving, so that is probably the equivalent of 600-800g per day. When I did this, I found myself snacking less on garbage because I had to get my veggies in and would eat carrot sticks or grapes or a banana instead of crackers or cookies. I ate more salads and always made sure there was a vegetable on the plate with dinner... so I saw immediate results in terms of weight. Now, it's hard to know what contributed more to gym performance... weight or nutrition... but I felt like my cardio and gymnastics improved... my guess is that the diet contributed some in terms of energy, but mostly having 10 lbs less to move through space was the bigger contributor.