Summary: I made a breakthrough the other day and installation with Fedora 23 regular USB media works, and after upgrading all packages (brings in kernel 4.4.6) dual monitor setup also works and appears stable. This is with XPS15 and Precision T3620. Disclaimer: I haven't yet tried installing to the NVMe-drive on the XPS (used an external drive so far).
Also note that to simplify I've completely disabled the drivers for the NVidia graphics and have tested only with the Intel drivers so far.
I have no information on the TB15 dock but gather from the forums things are not great yet.
In more detail:
Fedora 23 installation experience: Installed using boot LiveCD from USB stick. It goes to the GRUB boot menu from the installation media, here you can select to start the Live environment. I got one line of text, then after a few seconds delay complaints from xhci_hcd. If I did nothing it would then launch the boot animation, but after a while drop out saying it can't find the root filesystem. For me, the trick was, immediately when the first xhci_hcd complaint is shown to rip out the USB stick and then promptly insert it again. Then it booted the live environment successfully. This was the same on both the XPS15 and the T3620.
In the live-boot I think the mouse was a bit jerky, but it worked well enough to perform the real installation at least. After booting to the installed environment you will have the 4.2 kernel which seems able to bring the system up, with at least basic graphics.
Since you bought the laptop with Windows you probably have to change SATA mode to AHCI in bios before Linux will see the internal drive.
The laptop (XPS15) was the first I got, in late January, with Windows. At that time, the 4.2 kernel didn't produce any useful and stable result with external monitor connected. I made an attempt with the 4.3 and possibly early 4.4 but it wasn't good. Switched to Intel's latest drm-intel-nightly which improved things slightly but was still very unstable as soon as I connected external monitors.
The Tower 3620 arrived just last week. I bought it with Ubuntu preinstalled (to show up in the statistics), but currently prefer Fedora, with the same installation experience as above. Fedora 23's most recent kernel was now 4.4.6 but it too didn't work for me, and drm-intel-nightly was also horrible. (It was stable with 1 monitor, but not with 2.)
Then the magic breakthrough arrived finally! 🙂 During a bit more systematic testing I discovered the distribution kernel 4.4.6 does actually work on both the laptop and the tower, with dual monitors, provided they are connected with HDMI or DisplayPort 1.1 (i.e. if I enable DP 1.2/MST in the monitor settings, all *** breaks loose). The monitors used are Dell U2415H, possibly you could get different results with different monitors. The drm-intel-nightly kernel was still unstable even with MST disabled.
For the laptop I've used the fixed HDMI port and also a USB-C-to-DP converter. Now with 4.4.6 I can actually plug both in at the same time so there are in total three displays connected and working.