I've also been one to change my sensitivity a lot when I was doing bad and always legitimately did better afterwards. For a while I thought it was because the sens was better but when I kept changing in a cycle I realized since I was unfamiliar with the sens I had to focus more on hitting shots instead of relying on muscle memory. I don't think you did better after changing it because of placebo but rather because you had to focus on aiming more because you didn't have any muscle memory to back you up.
If I went from 9" to 14" I started focusing more on the crosshair and individuals rather than being aware of people and letting muscle memory take over. I think of accel similarly along these lines. Yes, it helps you at all distances but I'd want to never get used to it because the lack of muscle memory is what would help with accel the most imo. Sounds weird saying that though. I'd personally stay away from accel unless you're playing games that are drastically different in terms of what is needed from your mouse movement.