So now I have the 2017 MacBook Pro 15" with AMD Radeon RP560. It works with 2 External 4K Displays. It is really slow with the internal Intel Graphics Chipset. But you can deactivate automatic Graphics switching in System Settings -> Energy Saving. Then it is really usable.
I tried first with two Belkin Adapters (http://www.belkin.com/de/p/P-F2CU038/) for USB-C to HDMI. There I had some flickering problems on the Screens. Even with High-Speed-HDMI Cables (which are said to support 4k@60Hertz by the vendor).I can use these Adapters without any Problems with 2 DELL UltraSharp U2717D Displays at WQHD Resolution.
I also have two Wesimi USB-C to MiniDisplayport adapters (https://www.amazon.de/wesimi-DisplayPort-Adapterkabel-Aluminium-ChromeBook/dp/B06W54YF2P). I did not find any Manufacturer-Website for this product. That's why only the amazon link is provided. They work really great with the supplied cables from Dell (shipped with the Displays) at 4K resolution.
So now Youtube 4K Streams run smooth. And I use this Setup for my daily Work as Web-Developer. I haven't tried to use this setup for Gaming.