5K Display (only one) and MacBook Pro mid-2015 DG Radeon 370x Which 5k displays supports by my MacBook Pro mid-2015 with Radeon M370x and how it's done?
Is some one saw articles or tests about MBP 2015 and 5k displays?

Right now I'm replacing my two old fullHD 24" displays and after few weeks of tests I don't like two 4k 27" displays... I returned them.
I don't want two displays anymore...
I want one but ultrawide (21:9), for now I'm thinking about LG 38UC99-W with 3840x1600, but soon there will be new 5k ultrawide LG 34WK95U with resolution 5120x2160.
First LG 38" 4k works with my MacBook but if MBP works with 5k displays... there will be possibility that it will work with 5k Ultrawide...
Per the technical specifications, your MacBook will support a maximum resolution of 3840x2160 which is technically UHD 4K (16:9)

Can you connect a 5K display?

Yes, but it will show at the 4K resolution if you use one Thunderbolt cable.  If you use two, you can drive a 5K and the display must support MST (Multi Stream Transport).  In this scenario, only one external display is supported.

See Using 4K displays, 5K displays, and Ultra HD TVs with your Mac for more details.

Bottom line: you can only display up to the maximum that your source can push.  In other words, you can drive two monitors at 4K or one at 5K