To start with we can give you the specs of the SSD Apple used in your system it's a PCIe x4 (4 lane) custom SSD blade. Apple disclosed this when the system was intro'ed.
So far OWC is the only vendor that has created a compatible blade type SSD for the MacBook Air & Pro laptops. Sadly it's only a PCIe x2 device and it's not as fast as the Apple models. So far they have not offered any solutions for 27" iMacs (Late 2013 to current)
Even still I wouldn't swap out what you have or add it. The rub here is taking off the display and having to completely remove the logic board to gain access to the PCIe slot the SSD sits in. Many people have killed their displays in the process which is a very expensive mistake!
OK what to do here? Frankly you have another option which will get you very close to the same performance without opening the system!
I would get an external RAID SSD drive set using Thunderbolt 2. Here is what we use: