ok. 

one more question - if i hook up mbp via hdmi, then i can get 4k@60hz and high speed data transfer, right? 

Prior to the purchase, you should read through the U3223QE threads on the Apple Forum.

If I get it right, I can't have simultaneously High Speed transfer and run display 60Hz? I have to choose?

You have to choose one or the other. The U2723QX/U2723QE/U3223QE User's Guide is specific in the language concerning USB-C prioritization. It states "or", not "and" =

Allows you to specify the priority to transfer the data with high resolution (High Resolution) or high speed (High Data Speed) when using the USB-C port/DisplayPort.

Does that apply only to video and data upstream USB-C port?

Applies to only #5 USB-C port (Video + Data).

If I connect the other upstream port to laptop and hook let's say external hard disks to downstream ports, both USB-C and USB-A, do I get High Speed data transfer and 60Hz?

#8 USB-C upstream port is data only, no video. If you choose high speed for #5, you get high speed for #8.

thanks. it’ll work too if i hook it via displayport to thunderbolt dock like caldigit, i presume. 

It is HDMI 2.0 should work at 4K 3840x2160. The speed is not effected using HDMI or DP.