Re: 2560x1440 not working..
AFAIK, Dell U2711 (you have this model, right?) doesn't support 2560x1440 via DP at all (so mDP -> DP won't work). At least there are a lot of info of those who couldn't get it working via DP and no one who could.
You can get 2560x1440 only if your NB supports DP 1.2 or higher and only via mDP -> Dual-Link DVI-D converter (I don't know if your converter was Dual-Link, see wikipedia for connector pinout).
OS: Gentoo Linux, ~amd64. VideoCard: GeForce GTX 470. ASUS P8P67 Deluxe, 2x4GB DDR3-1600 CL9, Core i7 2600k.