m.2, yes.
Samsung XP941 and SM951(AHCI version) are bootable. Any dumb passthrough PCIe x4 carrier card will work.
Any NVMe m.2 drive, such as Samsung 950 PRO, Intel 600p, or Plextor M8Pe is not bootable. To use as NVMe as storage requires IONVMeFamily.kext, a 3rd party driver. Freely available, but requires a little effort to make it work.
Unless you need >512GB boot drive or what you're doing is very latency-sensitive to the point that NVMe would provide meaningful benefit, the 512GB SM951 AHCI is it. Part # MZHPV512HDGL . The drive will max out at 1340-1400MB/s read/write with latency that is second to none for a cMP boot drive.
You can find em for $200-250, lightly used by some PC gamer type if you have time and patience.
AHCI = cMP bootable
NVMe = cMP non-bootable