Not sure if this meets your needs, but I find Air Display to be excellent for this purpose. Zero quality degradation, and the latency is less than I expected. Q

Not sure if this meets your needs, but I find Air Display to be excellent for this purpose. Zero quality degradation, and the latency is less than I expected. Quite usable for most purposes. I grant you're not going to be doing video editing this way...but you could certainly use the iPad for the palettes for the app you're using to do the video editing.