Reasons to not use higher values than 6/11 in windows: It makes windows skip pixels, rendering it completely impossible to land on some pixels.
NEVER use above 6/11.
Reasons to use 1.0x in osu instead of other settings: if you use anything besides 1.0x, the mouse speed inside osu and outside of it in the rest of your PC browsing will be different, which might be awkward or annoying. It's not that hard to get used to but you'll probably still be better off trying to be consistent.
Reasons lower sensitivity is generally better than higher sensitivity: it makes your aim more accurate. Try to land on a pixel with any level of sensitivity, and it will be easy for you to tell that the higher it is, the harder it is to land on that pixel. Osu circles are bigger than a pixel but the same principle remains. It is a common misconception to think that higher sensitivity will make jumps easier, since a jump is a "big fast move", but in reality the difficulty in jumps isn't 'moving from point A to point B', it's 'landing on point B accurately and consistently'.
Reasons why too-low sensitivity is bad: to get the maximum speed on your aim, it is recommended to only use the wrist for aiming, and not the arm. Depending on the size of your wrist, each person has a different optimal sensitivity level for them, which is the lowest-yet-not-too-low level where they can move anywhere with their wrist. Also, this is trainable, so training yourself with a lower sensitivity than what you think you can play will make you better later (it will make your wrist stronger, and better at jumps).
A full-area tablet will require you to move approximately 3 inches from the left-most to the right-most edge of your osu playfield. This is equivalent to about 450dpi on mouse. Many people experience an improvement in their jump consistency when switching from mouse to tablet. In reality, they were most likely using a very high sensitivity with mouse (anything more than 450 pixels per inch) and by switching to tablet and not changing the default settings they started playing on a lower sensitivity which made them better.
Thus, if you want to have good consistent results with mouse, it is recommended that you just aim to set a sensitivity level of 450dpi and play with this for at least a month. This will make you instantly "jump as good as on tablet" (in reality it's just "jump as good due to lower sensitivity") and you will see a rapid improvement in your aim. After playing for at least a month with this setting, you might want to make it a little lower or higher based on your own wrist, once you already learned how well this works for you.
It's not impossible to learn to be good with higher sensitivity levels, you'll just need to train more and work harder for smaller/slower results, so there's really no point unless you particularly find higher sensitivity more fun to play with.
And it goes without saying, make sure any form of acceleration is disabled. Triple-check that you're sure on this. i.e. confirm that doing fast moves doesn't move you further than doing slow moves for the same distance.
Personally I find lower more fun. 320dpi masterrace gogo. QCK+ wisest purchase ever