4k display scaled to 1440p. Will a newer MBP have far better performance?

[circa7]

I currently have a 2015 rMBP, stats here:

• MacBook Pro (Retina, 15-inch, Mid 2015)
• 2.5 GHz Intel Core i7
• 16 GB 1600 MHz DDR3
• AMD Radeon R9 M370X 2 GB

I recently purchased an LG 27UD69 4k display and it works fine at 60hz for general use, but if I use any semi-intensive applications (like Sketch or Photoshop), my system lags too much to be usable when I have it scaled to 2560x1400 (native resolution is just too tiny).

Connected to my Apple Thunderbolt Display it works fine. I realize this is probably because of the increased demand from the scaling... so my question is, if I buy a 2016 or 2017 rMBP with a newer, 4GB GPU, will it be able to handle the increased demand?

I'm also using a 10 foot displayport cable... wondering if buying a shorter one would help.

Thanks guys!