I think you missed the MBP in what I outlined. People have been selling the much easier to get their hands on MBP blades as MP blades upgrades. It is doubtful that many real MP 2013 blades are floating free. Apple isn't selling them. There are no 3rd parties to upgrade to and sell the old. Apple service vendors selling of rogue spare parts... not sure how that wouldn't set off alarms at Apple for when almost every deployed Mac Pro is practically brand new.
A x2 lane PCIe v2 SSD isn't going to match a x4 lane SSD v2 even if the socket, flash controller and flash are identical. Likewise there can be firmware and cache differences even if plug into the same socket.
The marketing page for Mac Pro (
http://www.apple.com/mac-pro/ ) has a "Storage section". Max SSD transfer time 1200MB/s . A different value than 750 and 650 MB/s. The MBP doesn't really have x4 PCie v2 lanes of bandwidth to completely dedicate to a SSD so there is little reason to put a 1200MB/s SSD in one. The Mac Pro 2013 has substantially more PCIe bandwidth so it does make sense there.