For those that have had issues getting this work with the later version of the mbp and el capitan, specifically, the ssd not booting after the virtualbox restart step, I found I needed to go a different way and using different instructions, but hopefully this helps someone.

Fwiw, I was installing Windows 8.1 and then using the free Windows 10 upgrade.

The steps were to follow the instructions here:

However, when it comes to the disk partitioning steps, I used the following (original source https://msdn.microsoft.com/en-us/windows/hardware/commercialize/manufacture/desktop/configure-uefigpt-based-hard-drive-partitions and I modified the msr size to 150MB):

rem == CreatePartitions-UEFI.txt ==
rem == These commands are used with DiskPart to
rem    create four partitions
rem    for a UEFI/GPT-based PC.
rem    Adjust the partition sizes to fill the drive
rem    as necessary. ==
select disk 0
clean
convert gpt
rem == 1. System partition =========================
create partition efi size=100
rem    ** NOTE: For Advanced Format 4Kn drives,
rem               change this value to size = 260 ** 
format quick fs=fat32 label="System"
assign letter="S"
rem == 2. Microsoft Reserved (MSR) partition =======
create partition msr size=150
rem == 3. Windows partition ========================
rem ==    a. Create the Windows partition ==========
create partition primary 
rem ==    b. Create space for the recovery tools ===
shrink minimum=500
rem       ** NOTE: Update this size to match the
rem                size of the recovery tools 
rem                (winre.wim) plus free space                   **
rem ==    c. Prepare the Windows partition ========= 
format quick fs=ntfs label="Windows"
assign letter="W"
rem === 4. Recovery tools partition ================
create partition primary
format quick fs=ntfs label="Recovery tools"
assign letter="R"
set id="de94bba4-06d1-4d40-a16a-bfd50179d6ac"
gpt attributes=0x8000000000000001
list volume
exit

Good luck