M.2 on older motherboard coected via PCI-E adapter

I'm looking into getting a Samsung 950 Pro 512 GB SSD with read at 2500 MB/s and write at 1500 MB/s. My motherboard is an Asus Sabertooth x79 so it does not have an M.2 slot on it. I was wondering if I used a M.2 to PCI-E adapter if I would get the above read/write speeds or if there would be a bottleneck of some sort? Further, if I can achieve those purported speeds, is there anything I should pay particular attention to when getting the PCI-E adapter?