PCIe adaptors with just one blade, the maximum possible is around 1450MB/s, but you can have multiple blades on a Amfeltec Squid or Highpoint 7101A and get 4x this, if you have deep pockets.
Take a look on these threads:
Amfeltec x16 PCIe with 4 SSDs: 5900+ MB/s
Highpoint 7101A - PCIe 3.0 SSD performance for the cMP
On my home Mac Pro, my main disk is a PCIe Samsung SM951 512GB + Angelbird Wings with a SanDisk Extreme as scratch disk. SM951 gets a little over 1440MB/s read/write. It's a somewhat costly option, SM951 it's a AHCI drive, but the safest and trouble free one. I bought mine on eBay after some months searching for a new one with a decent price, $245 in February, it's higher now.
On my office Mac Pro, I have a no name Chinese PCIe adaptor with a Apple/Samsung SSBUX 512GB, ~1450 read/1375 write. I don't like these cheap adaptors, my home Angelbird Wings never gets hot unlike the Chinese one. I will have to install heatsinks on the SSD blade before the start of the summer here and I really don't recommend this route, unless you need a 1TB AHCI drive.
The biggest SM951 is 512GB, but Apple has a SSBUX version with 1TB. Basically it's the same drive/same controller, since both are made by Samsung, but Apple has different firmware and the pinout of the connector is proprietary, so you don't have options to choose PCIe adapters like on the standard one.
If I was gonna upgrade today, I'd still buy the Angelbird Wings PCIe card but with a Samsung 970EVO 1TB. A little more expensive than the 512GB SM951 but double the space and same performance. It's a NVMe drive, so you have to add NVMe driver on the BootROM.
