If your primary concern is Linux support, then look to the places where you can buy Linux laptops:
- Dell "Developer Edition": http://www.dell.com/learn/us/en/555/campaigns/xps-linux-lapt...
- others??
For me, the primary metrics are: excellent keyboard, touchpad WITH buttons (limited options nowadays), portable size/weight. It sounds like you're more concerned with performance, so you probably need to be in the 14"+ category.
Also the OP asked for 4 cores, the U-range has just 2.
You can install coreboot on other machines, there will still be closed source microcode in the CPU and closed firmware in some of the controllers.
And the hardware is simply out of date, you can get a dell xps 13 with more modern hardware for the same price and in essence the same level of assureance.
If you are worried about state level actors exploiting bugs or backdoors in your hardware Librem isn't what is going to stop them.
If you want want somewhat less quality and half the price or so I'd look at the lenovo 710s.
Aluminum+magnesium chassis, large touchpad, iris 540 graphics, i7, and 1080P matte display. Some might consider 1080P a big downgrade, but for a small 13.3" display I think it's quite reasonable.
i7-6560U, iris 540, 16GB ram, 512GB ssd = $1,199.99 (free shipping).
The most comparable macbook pro is the base, dual core i7 upgrade, 16GB upgrade, and 512GB upgrade = $2,199. That's without the dongles you need.
With the 710s you can drive a TV/projector with HDMI, read a SD card, charge a phone with a normal usb cable, etc. No fancy display on the keyboard and no soft keys to replace esc.