TB15 + U2715K + Precission 5510 still can't reach 5K

I was told by Dell that 5K support for the Precission 5510 + U2715K + TB15 dock combination was in the works, and they were waiting for intel drivers to sort out flickering, etc. issues when the monitor is connected by dual displayport cables in 5K mode. I just updated to the 1.2.1 BIOS, and updated the thunderbolt controller drivers, firmware, intel graphics drivers, etc. that were very recently released. It appears I can now use dual cable connected 5K mode properly, but only if the resolution in Windows is set to 4K (previously, this would distort heavily). And USB, etc. seems a lot more stable. However, if I turn the monitor resolution to 5K, the image looks very low resolution, stretched, "wide". It's not flickering, and there's now half-half as before...but the image is stretched, and unusable. Is there some workaround, or do we need to wait longer for 5K to work?