Thankfully, I found that the XPS13 has a DisplayPort output that is capable of driving two daisy-chained monitors in addition to the internal screen (no mirroring!), even at 2560x1440 on each. I use it with two Dell 27" monitors. They must support DP1.2 for chaining to work. To be fair, there was a regression for hotplug detection from Ubuntu 15.10 to 16.04, but it was recently fixed.