I highly recommend Lenovo for the powerful machines. I love my T450s — 20GB RAM, 6th gen low voltage Intel CPU, half a TB SSD, and a 940m nVidia.
They have refreshed the range less than a year from when I bought mine, with T460s and T460p (s = lighter, for travel, p = performance, proper quad-core CPU, 32GB RAM etc.)
And to top it off, Lenovo has some of the best engineered machines for cooling in the PC world.
I do realise I’m starting to sound like a fanboy, but I spent many months searching for a proper development computer under 1.8kg with a 10 hour battery life, and I was disappointed by the power provided by nearly every other manufacturer.