My experience on a TB 13" MBP:
Received the screen earlier today and had a chance to finally test it out this evening with my maxed out TB 13" MBP.
Reading some of the feedback from other users here (and thanks to everyone who have been sharing their experience), I was quite concerned with the performance I would get through my 13" but I'm happy to report to all seems to be working quite smoothly for me. No real lag during regular usage (incl. mission control animation) and I tested this both through the stock cable and my Belkin 0.5m TB3 cable.
I also played a few 4K videos via Safari (i.e. youtube.com/watch?v=RLOR6EX8Nfw) and again, super smooth, no lag whatsoever. Fans didn't even kick in. I did notice that when playing 4K videos via Chrome, my CPU usage skyrockets, but no issues with Safari.
Working around in Lightroom did engage the fans but it was on and off (i.e. when scrolling through my library, quickly jumping between images fans came on but once an image was selected and I started editing, fans slowed/went quiet).
I did notice a weird bug with my mouse. When moving my mouse quickly and the 'large mouse' animation appears, it stayed 'stuck' on the display. This has happened twice so far and unplugging and replugging the monitor solves this.
Resizing windows in Sierra is smooth though this is not always the case with MS Office. I think that the lag is because of software vs. hardware as in Outlook, resizing from the message view works well but it is much choppier in the calendar view.
I'm streaming a hockey game right now via Gamecentre Live and everything is running smoothly though the quality looks awful if I go full screen (no fault of the monitor). Chrome is eating up 80% of my CPU to stream the game (flash) and the fans have kicked in.
All in all, I'm super happy with the set up. I was fully preparing to have to go through an exchange process to get the 15" but based on my experience so far, I will be keeping my 13".
*EDIT* Just remembered I can enable Flash in Safari. Loaded the hockey game on Safari instead and CPU is now virtually nothing.