I've used phenibut, 1g before sparring. It was maybe the best sparring session i've ever had. I was completely calm thus much more clearheaded and was able to throw lots of combos exhausting myself physically but never feeling pressured. I was much more receptive to my coaches words too, the queues for punches were clear and my response was quick as well.
I love it, but won't take it more than 2-3 times a year. That is overly-cautious but i dont want to build tolerance