That guy is me, with two bits of feedback. TL;DR - confirmed. You will need 10.10.3 to use two displays as I have.
First, I have two Dell P2715Q displays and can get up to 4k @ 60Hz on both displays simultaneously (in either portrait or landscape or a combo of positions) while driving the built-in display. I recommend a quality cable - I know what the DisplayPort.org web site says, but I have had a better user experience with DP 1.2 2160p cables - they're not easy to find (we're using both StarTech and Lindy cables).
Second, for the Mac we're discussing here, Apple must have recently revised that web page - and it's not correct in regard to specs for types of displays. The first relevant group of Macs, "Supported displays and configurations" lists the late-2013 rMBP (which I have, with the 750M).
The second relevant group of Macs, "Single-Stream (SST) displays" is not necessarily accurate, and I cannot confirm the accuracy of the third relevant group of Macs, "Multi-Stream Transport (MST) Displays" - my Dell displays ARE MST displays - but they only work in SST mode and will not work in MST mode. Dell shows in its manual, and CS confirms that MST will work with a DP>DP (not mDP) cable connected to the PC leading to the primary display, with a DP>DP cable connected to the primary display leading to the secondary display. I tried MST with a rMBP - mDP>DP cable - primary display- DP>DP cable (both cables are DP 1.2 2160p certified), and I would see 4k @ 30Hz on the primary display and a dark screen on the secondary display.
The caveat for the MST layout - I'm using RGB 4:4:4 on both displays (I used an EDID script to create a unique DisplayID profile for each display so that they're recognized as DisplayPort displays and not "Televisions"), which I've been told uses more bandwidth than YPbPr. Both displays are working flawlessly in SST mode - with crystal clear text and images.
