If you've checked OWC's website you would see that so far they have not released a PCIe based SSD for these Macs.
Whilst the industry is moving towards PCIe SSD sticks for notebooks and even desktop motherboards now include M.2 slots which are a kind of PCIe SSD slot there is a problem.
Apples connector that their SSD sticks use appears to be custom. So due to it not being slot compatible with the industry standard it limits the market.
On top of this there are some unknowns to the public. Although Apples connector is different we cannot assume that electrically it is the same as the industry standard with just the pins rearranged in a different configuration.
It may be the case that Apples implementation uses different voltages and may have less or more pins. If this is the case it could make their PCIe SSD sticks incompatible with the chipsets that are available on the market requiring custom silicon just for an Apple compatible stick.
This would be different to the previous SATA stick used in the 2012 Retina MacBook Pro's as those were voltage compatible with the industry standard and so the same SATA chipsets could be used with only the connector itself needing some of its pins re-arranged.
In the post you linked to, OWC showed off a prototype in January 2014. Well it is now June 2014 and it has not gone in to production. I do believe we will eventually get a compatible stick if Apple doesn't change the setup again with the next model.
But there is a big issue, they may do just that to move away from AHCI to NVMe which is a new specification for communicating with flash memory. The industry is slowly moving from AHCI to NVMe because it offers much greater performance for Solid State Storage access. If Apple moves to this technology it will cause a headache for 3rd party SSD manufacturers because they will now need to offer SSD sticks for 2012 MBP's 2013 MBP's and 2014-2015 MBP's that require NVMe SSD's.
This further fragmentation could harm the 3rd party Apple SSD market just as much as moving to the proprietary PCIe connector has.
So in closing, save up and buy the amount of SSD storage you will need. The future for 3rd party replacements is uncertain.
