I mean the output.
set up to take 12V raw,
So if you feed them with 10V they will be dimmer.
Also, am i right in saying that the MOSFET is either on or off?
Not quite, a FET has a threshold voltage but there is also a liner region that can half turn a FET on. Even with so called logic FETs. If you operate the FET in this mode it will have a large current down it with a largish voltage across it and you will exceed the power dissipation of the FET.