They dont reduce tearing. Only V Sync can do that.
The rest is placebo. As i have said your reaction times are 250ms give or take so 8ms wont affect anything. In Fact its well within the margin for error of the human response time.
Vsync fixes all stuttering, frame time variances which show speeding up and slowing down of game play and all tearing for a near perfect game play experience. This is FACT not placebo. You can get all these benefits from just 60 FPS. If you need sub 40ms response times then Vsync with 120hz is the answer.
No it does reduce the appearance of tearing, as the frames are kind of synced with the harmonic frequency of the refresh rate at those caps.
Tearing Interaction between fps and Hz (Harmonic Frequencies)
another consideration, if you got a powerful GPU that runs capped out at a game's framerate limit (e.g. source engine games with a configurable fps_max) .... then if you play VSYNC OFF, you _really_ do not want a frame cap (fps_max) that has harmonic frequencies between fps and Hz. For example, an fps_max of 60, 120 or 180 for a 60Hz display, especially if you have a graphics card (e.g. Titan). The reason is you will have nearly-stationary or slowly-moving tearlines, as the splice of the previous frame cuts into the next frame at fairly synchronized intervals. For example, I can see two very clear (nearly-stationary) tearlines during fast turns in an older Source Engine game (without AA) when I configure it to a fps_max 240 on my 120 Hz display, because my GTX 680 can easily run capped-out at 240 fps. And likewise, you will have more visible tearing on a 60 Hz display if you cap at 59/60/61 (one persistent tearline) or at 119/120/121 (two persistent tearlines) or at 179/180/181 (three persistent tearlines) assuming your GPU is powerful enough to always run fully capped out at the fps_max. Instead you prefer tearing to faintly & randomly go all over the place rather than being obnoxiously stationary or in the middle of your screen. Uncapping is better (e.g. fps_max 999), or setting an odd number as high microstutter-free value as you are able to (e.g. fps_max 317), can significantly reduce the appearance of tearing.
The only things he fails to mention is that the best numbers to use are 125, 250 and so on, instead only citing 317.
Secondly, vsync does not automatically fix stuttering in MGPU, I know this from first hand experience and I actually asked on another forum why that is. This is the response I was given from someone who deals with AMD on a adaily basis, assists or attends their press events, and also happens to review for the ATi fan site, Rage3 D
timing.
vysnc doesn't time the start of the frame render, it adjusts the timing of the display of finished render.
so when have an offset of say 5ms between GPU1 starting a frame and GPU2 starting a frame, and displaying every 16.6ms, you've got GPU 2 rendering a frame based on actions that happened before you saw frame 1. By the time you see and react to frame 2, it's 33ms since frame 1 began rendering.
This is why its advised to run a frame cap in conjunction with vsync in troublesome games.
Thirdly, vsync at 120hz still produces noticable input lag. Yes its much better than 60hz but its still there and noticable.
So.. now that you have learnt some facts, I think its time to move on.