I have 13" 2014 rMBP replaced the original 128 SSUAX x2 ssd with 512 SSUBX x4 ssd and can see 1400/1400 read/write speeds. If your SSD supports x4 then it should work. Do you have kapton tape on adapter?
According to Samsung my SSD does indeed support x4 and while my adapter did come shielded from the manufacturer I've added some tape but it's still only operating at x2.