After reading peppy blog, I actualy thought something has been done about optimus s (As peppy is asking about perf issues by optimus s), so I tried switching osu! to dedicated GPU (Because with the intel one I must play in 720p resolution, when I could play in 1080p using DirectX on dedicated GPU before) and...
Actualy I feel like it's the exact same as 4 mounths ago when I was still playing, high input lag and very bad performance overall depsite usng 720p (Intel GPU give better perfs, this doesn't sound right)
I think "dedicated" full screen still doesn't work correctly.
I have F.lux on my PC too, but depsite osu! saying me that F.lux makes osu! performance bad, it doesn't seem to affect me when playing full screen with integrated GPU (And I can't desable it or my eyes are going to be broken after 5 minutes, too.)
EDIT : I tried disabling F.lux and perf was better, but it still has input lag, "dedicated"' fullscreen definitly doesn't work as you can see below (Something like this can't happen on dedicated fullscreen)

You can't get the taskbar over a game that is in true fullscreen.