I use my blackwidow like a piano, my arm doesn't rest on any surface, and the keyboard is fairly close to me too. Extending my arm slows down my stream speed
I smash the keys pretty hard and do a full "press and lift" key-press, even on slower streams. The key point I have noticed is consistency in using the same amount of force between both fingers, otherwise the 1 that hits harder slows the other down. (In my case, the middle finger, since it's longer and stronger.) Also I like building stamina no matter what stream speed it is lol... smashing at 150bpm.....
I found it easier to find the right 'force' by keeping my arm in the air. Dunno that anyone has actually talked about this before, is this ideal? I've had success at nailing 300's at 200-220 bpm much more consistently with this method.
As for keyboards: Anyone know if the coolermaster storm quickfire rapid (Browns) keyboard is any good for osu!? It seems to be 'okay priced' in my country, slightly cheaper than the blackwidow even. Brown switches seem to make a lot of sense too, light and tactile, and function a bit like reds.