5K external screen with 2015 15 inch retina MBP with Intel Iris Pro graphics Has anyone tried whether Intel Iris Pro found in 2015 retina MBP is powerful enough to drive single 5K screen like Dell UltraSharp UP2715K or Iiyama ProLite XB2779QQS or Philips 275P4VYKEB or HP Z27q? (basically all the ones that aren't relying on usb c/thunderbolt 3 connector like LG 27MD5KB-B UltraFine)?

I'm trying to find an external screen for my MBP and unfortunately due to macOS/OSX's focus on retina screens, non retina screens (fonts mainly) look pathetically bad, whereas Windows (even launched within OSX via RDC) looks sweet. So the only option is to either go 4k but that means usable resolution would be 1080p or 5k with usable 1440p. Hence I'm wondering if it can drive 5k screens.
No, the mid-2015 15" Retina MacBook Pro is not powerful enough to drive a 5k-display.

You can see the specifications here:

Mid-2015 Retina MacBook Pro Technical Specifications

You're limited to driving up to a 3840x2160 pixels external display via the Thunderbolt/mini-DisplayPort port while using the built-in display. 

For the HDMI video output you can go up to 4096x2160, but only at 24 Hz. So still no 5k support.