The Overlay settings are activated once you hook up to a TV via S-Video and start playing a video that has some overlay settings (just about all DVDs have this, for example). For everyday use of only the Thinkpad's LCD, they are grayed out.
As for the screen brightness, well...That varies amogst models even in the same series, much less one from one T4X to an X4X. I can't really tell because my friend runs his X40 at full brightness nearly all the time; he still gets great battery life anyway, so a smaller screen cranked up to max is going to suck less juice than a large screen cranked up to the "same" intensiy.
HTH
