Apple has begun to support lower-cost UltraHD displays with the release of OSX 10.10.3.
Previously, Apple’s devices would only send UltraHD signals to Multi-Stream Transport displays at 60Hz. With this change Single-Stream Transport models, like Dell’s UP2715K, are also supported at 60Hz. SST models tend to be lower-cost than those with MST technology.
Apple has officially announced SST support in various Macbook, Mac and iMac devices, produced from late 2013 onwards. They include the 13″ Macbook Pro (2015) and 15″ Pro Retina (2014), Mac Pro (2013) and Mac Mini (2014), 27″ iMac (2013 onwards), Macbook Air (2015) and 12″ Macbook Retina (2015).