RE: XPS 13 9343 and 9350 QHD+ No Display Issues on Linux
Had you tried the Ubuntu 15.10 live CD at all?
I got my 9350 yesterday and got right to setting up Arch on it, but hit this exact issue. There is an AUR build script for the drm-intel-nightly build as well, however when I tried that kernel, it wasn't initializing the nvme drive, and when it dropped me into the rescue terminal, the keyboard wasn't working either. So that seems like a no go.
But on a whim, I tried booting Ubuntu off a USB disk and X loaded just fine on it, full resolution.
Kind of left scratching my head.
I love Arch, but contemplating Ubuntu for now just to get around this.