Dell P2415Q and LS27D85KTSR/ZA for 2015 Macbook

The best I can get is 30 bit colour, 30Hz at 1692 x 3008 (15" MBP touchbar Radeon Pro 450 2048 MB).

The best contrast is with hdmi and multiport adapter. There is shadowing using Display Port to usb-c and MST.

I am amazed that Dell only support Windows.

It will support 4K, but only at 30hz.

Using 4K displays and Ultra HD TVs with your Mac - Apple Support

You will need a USB-C to HDMI adapter.

http://store.apple.com/us/product/MJ1K2AM/A/usb-c-digital-av-multiport-adapter