2015 MacBook Pro: Only the High-End Model supports 5120 X 2880 External Display
Authorized Apple dealer B&H Photo loaned MPG the Apple 15.4" MacBook Pro Retina (Mid 2015), 2.8GHz / Radeon R9 M370X / 16GB / 1TB, which MPG recommends for power users, photographers, videographers, etc. Save money on AppleCare too.
See the MPG review of the 2015 MacBook Pro.
As per Apple specifications, it would be wise to go right to this top-end MacBook Pro Retina model if there is any goal of having an 5K display (please use that link so that MPG gets credit).
MPG intends to test the Dell UP2715K 27" UltraSharp 27 Ultra HD 5K LED Backlit IPS Monitor on this 2015 MacBook Pro.
Thunderbolt digital video output
• Native Mini DisplayPort output
• DVI, VGA, dual-link DVI, and HDMI output supported using Mini DisplayPort adapters
• Support for up to 5120-by-2880 resolution at 60Hz on a single external display (model with AMD Radeon R9 M370X only)HDMI video output
• Support for 1080p resolution at up to 60Hz
• Support for 3840-by-2160 resolution at 30Hz
• Support for 4096-by-2160 resolution at 24Hz
