2560x1600 is the most it can really drive, IMO.
Unfortunately, those displays are twice the price of a 4k display.
Hopefully, Apple will come out with a 5120x3200 display at some point (and a MacMini to drive it), so I can have the same screen-size and screen real-estate again, when I upgrade.
2560x1440 monitors aren't a bad substitute for the money.