maybe the screen tearing is amplified since there's more frames being drawn per second?
It is not amplified. When fps are multiple of display frequency, tearing occurs in roughly the same places, fps are still smooth, but images have several evident stripes. When fps are high like 500 and not multiple of refresh rate, tearing is almost unnoticeable.
capping @ 600 fps on a 60hz monitor gives me screen tearing, and it's completely unplayable unless i turn off multipoll
It is a mingw problem. It tries to mimic linux-specific features, but this is very unstable. This can cause different very weird things.
Instead of promoting certain settings that work on an individual's client, I believe we should be encouraging players to experiment until they find settings that work for them.
Absolute no. Experiments are ok with look and feel settings, but not with some buggy and obscure system stuff in default community client. Nobody here understands what multipoll does. It should be smth like: higher fps -> less lag, no vsync, no any "magic" settings.
As for SDL2, it has only relative mouse, it may be a raw input (unaffected by additional os processing, like adding acceleration or dependence on mouse speed settings), but it is not smth guaranteed. There should be a way to get actually used api/mouse move event rate.
Also SDL2 fullscreen used in client is just a fullscreen window, so it can't change display resolution/refresh rate. It may be a good thing -> better window switching, but it may be also affected by os composing wm, with extra lags/fps drop.
And in short, if client can't offer a good base for all platforms (including performance, input or wm integration), adding some minor tweaks to is worthless.
P.S. Consider this as some feature requests to Star's client.